feat: add multi-language (i18n) support with Japanese UI

apps/solvecraft/src/main.rs

@@ -169,6 +169,7 @@ fn main() -> eframe::Result {
         "SolveCraft",
         options,
         Box::new(move |cc| {
+            solvecraft_ui_egui::fonts::setup_fonts(&cc.egui_ctx);
             let mut app = SolveApp::new(Session::default(), services());
             app.custom_titlebar = CUSTOM_TITLEBAR;
             app.integrated_titlebar = cfg!(target_os = "macos");

crates/ui-egui/src/browser.rs

@@ -351,7 +351,8 @@ pub(crate) fn draw_row(ui: &mut egui::Ui, id: egui::Id, row: &Row) -> RowResp {
         x += 21.0;
     }
     let col = row.color.unwrap_or(if row.dim { t.text_dim } else { t.text });
-    let galley = ui.painter().layout_no_wrap(row.label.to_string(), FontId::proportional(12.5), col);
+    let label_display = crate::i18n::tr_dyn(row.label);
+    let galley = ui.painter().layout_no_wrap(label_display, FontId::proportional(12.5), col);
     let lw = galley.size().x;
     ui.painter().galley(pos2(x, cy - galley.size().y / 2.0), galley, col);
     let mut bx = (x + lw + 6.0).min(r.right() - 16.0);
@@ -362,9 +363,9 @@ pub(crate) fn draw_row(ui: &mut egui::Ui, id: egui::Id, row: &Row) -> RowResp {
     }
     crate::scenario::publish_handle(&format!("row:{}", row.label), r.center());
     let tip = match hover_on {
-        1 => Some(if row.fold == Some(true) { "Collapse" } else { "Expand" }),
+        1 => Some(if row.fold == Some(true) { crate::i18n::tr("Collapse") } else { crate::i18n::tr("Expand") }),
-        2 => Some(if row.eye == Some(true) { "Hide" } else { "Show" }),
+        2 => Some(if row.eye == Some(true) { crate::i18n::tr("Hide") } else { crate::i18n::tr("Show") }),
-        3 => Some(if row.radio == Some(true) { "Active component" } else { "Activate" }),
+        3 => Some(if row.radio == Some(true) { crate::i18n::tr("Active component") } else { crate::i18n::tr("Activate") }),
         _ => None,
     };
     let resp = match tip {

crates/ui-egui/src/context_menu.rs

@@ -1273,7 +1273,8 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                 painter.circle_filled(c, 3.0, t.text_dim);
             }
             for (k, item) in radial.iter().enumerate() {
-                let galley = painter.layout_no_wrap(item.label.clone(), font.clone(), if item.enabled { t.text } else { t.text_dim });
+                let display_label = crate::i18n::tr(&item.label);
+                let galley = painter.layout_no_wrap(display_label.to_string(), font.clone(), if item.enabled { t.text } else { t.text_dim });
                 let size = vec2(galley.size().x + 40.0, 28.0);
                 let rr = slot_rect(c, k, size);
                 let resp = ui.interact(rr, ui.id().with(("radial", k)), Sense::click());
@@ -1295,7 +1296,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                     picked = Some(item.clone());
                 }
                 if item.enabled {
-                    resp.on_hover_text(item.shortcut.clone());
+                    resp.on_hover_text(crate::i18n::tr(&item.shortcut).to_string());
                 }
             }
         });
@@ -1393,7 +1394,7 @@ pub fn list_row_lit(ui: &mut egui::Ui, item: &Item, lit: bool) -> Option<Item> {
         ui.painter().text(
             pos2(r.left() + 8.0, r.center().y + 2.0),
             Align2::LEFT_CENTER,
-            item.label.to_uppercase(),
+            crate::i18n::tr(&item.label).to_uppercase(),
             FontId::proportional(10.5),
             t.text_dim,
         );
@@ -1415,9 +1416,11 @@ pub fn list_row_lit(ui: &mut egui::Ui, item: &Item, lit: bool) -> Option<Item> {
         );
     }
     let text = if item.enabled { t.text } else { t.text_dim.gamma_multiply(0.75) };
-    ui.painter().text(pos2(r.left() + 32.0, r.center().y), Align2::LEFT_CENTER, &item.label, FontId::proportional(13.0), text);
+    let display_label = crate::i18n::tr(&item.label);
+    ui.painter().text(pos2(r.left() + 32.0, r.center().y), Align2::LEFT_CENTER, display_label, FontId::proportional(13.0), text);
     if !item.shortcut.is_empty() {
-        ui.painter().text(pos2(r.right() - 10.0, r.center().y), Align2::RIGHT_CENTER, &item.shortcut, FontId::proportional(11.5), t.text_dim);
+        let display_shortcut = crate::i18n::tr(&item.shortcut);
+        ui.painter().text(pos2(r.right() - 10.0, r.center().y), Align2::RIGHT_CENTER, display_shortcut, FontId::proportional(11.5), t.text_dim);
     }
     (item.enabled && resp.clicked()).then(|| item.clone())
 }

crates/ui-egui/src/dialogs.rs

@@ -745,9 +745,10 @@ pub(crate) fn field(ui: &mut egui::Ui, s: &mut String) -> bool {
 
 pub(crate) fn combo(ui: &mut egui::Ui, id: &str, labels: &[&str], sel: &mut usize) {
     let row = ROW.with(|r| r.borrow().clone());
-    let r = egui::ComboBox::from_id_salt(id).selected_text(labels.get(*sel).copied().unwrap_or("")).width(FIELD_W).show_ui(ui, |ui| {
+    let selected = labels.get(*sel).copied().map(tr).unwrap_or("");
+    let r = egui::ComboBox::from_id_salt(id).selected_text(selected).width(FIELD_W).show_ui(ui, |ui| {
         for (i, l) in labels.iter().enumerate() {
-            let item = ui.selectable_value(sel, i, *l);
+            let item = ui.selectable_value(sel, i, tr(l));
             crate::scenario::publish_handle(&format!("dialog:{row}:{l}"), item.rect.center());
         }
     });
@@ -757,6 +758,11 @@ pub(crate) fn combo(ui: &mut egui::Ui, id: &str, labels: &[&str], sel: &mut usiz
 thread_local! {
     /// The label of the dialog row being laid out (its widgets publish `dialog:<label>` handles).
     static ROW: std::cell::RefCell<String> = const { std::cell::RefCell::new(String::new()) };
+    static LANG: std::cell::RefCell<String> = const { std::cell::RefCell::new(String::new()) };
+}
+
+fn tr(text: &str) -> &str {
+    crate::i18n::tr(text)
 }
 
 /// A dialog row's widget, for scenarios and demos: the handle `dialog:<row label>`.
@@ -848,11 +854,12 @@ const LABEL_MAX_W: f32 = 150.0;
 /// A row label in the dialog's label column (the grid sizes the column to its widest label).
 pub(crate) fn row_label(ui: &mut egui::Ui, text: &str) {
     ROW.with(|r| *r.borrow_mut() = text.to_string());
-    let full = egui::WidgetText::from(text).into_galley(ui, Some(egui::TextWrapMode::Extend), f32::INFINITY, egui::TextStyle::Body).size().x;
+    let display_text = tr(text);
+    let full = egui::WidgetText::from(display_text).into_galley(ui, Some(egui::TextWrapMode::Extend), f32::INFINITY, egui::TextStyle::Body).size().x;
     let w = full.clamp(LABEL_W, LABEL_MAX_W);
     ui.allocate_ui_with_layout(vec2(w, 22.0), egui::Layout::left_to_right(egui::Align::Center), |ui| {
         ui.set_width(w);
-        ui.add(egui::Label::new(text).wrap());
+        ui.add(egui::Label::new(display_text).wrap());
     });
 }
 
@@ -1026,9 +1033,9 @@ fn input_rows(d: &mut Dialog, ui: &mut egui::Ui) {
             // A chip: "↖ Select" (outlined) while empty, "N selected" (filled) once picked.
             let filled = !inp.items.is_empty();
             let text = if filled {
-                RichText::new(format!("↖ {} selected", inp.items.len())).color(Color32::WHITE)
+                RichText::new(format!("↖ {} {}", inp.items.len(), tr("selected"))).color(Color32::WHITE)
             } else {
-                RichText::new("↖ Select").color(t.text)
+                RichText::new(format!("↖ {}", tr("Select"))).color(t.text)
             };
             let stroke = if active { Stroke::new(1.5, t.accent) } else { Stroke::new(1.0, t.dialog_border.gamma_multiply(1.6)) };
             // Not in the Tab order: Tab moves between values.
@@ -1208,6 +1215,7 @@ fn auto_operation(app: &SolveApp, d: &mut Dialog) {
 /// the view cube, sized to its content.
 pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     let Some(mut d) = app.dialog.take() else { return };
+    LANG.with(|l| *l.borrow_mut() = app.preferences.language.clone());
     auto_operation(app, &mut d);
     update_measure(app, &mut d);
     update_fill_cells(app, &mut d);
@@ -1220,7 +1228,8 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     let mut cancel = false;
     let mut enter = false;
     let wide = d.is_wide();
-    let heading = if d.editing.is_some() { format!("EDIT {}", title(&d.kind)) } else { title(&d.kind).to_string() };
+    let raw_title = title(&d.kind);
+    let heading = if d.editing.is_some() { format!("{} {}", tr("EDIT"), tr(raw_title)) } else { tr(raw_title).to_string() };
     let frame = egui::Frame::window(&ctx.global_style())
         .fill(t.dialog_bg)
         .stroke(Stroke::new(1.0, t.dialog_border))
@@ -1807,17 +1816,19 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
             let buttons = if has_ok { 64.0 * 2.0 + ui.spacing().item_spacing.x } else { 64.0 };
             crate::frame::right_aligned(ui, total, buttons, |ui| {
                 if has_ok {
-                    let label = if matches!(d.kind, Kind::ConfirmDelete { .. }) { "Delete" } else { "OK" };
+                    let raw_label = if matches!(d.kind, Kind::ConfirmDelete { .. }) { "Delete" } else { "OK" };
+                    let label = tr(raw_label);
                     let b = egui::Button::new(RichText::new(label).color(if valid { Color32::WHITE } else { t.text_dim }))
                         .fill(if valid { t.accent } else { Color32::TRANSPARENT })
                         .min_size(vec2(64.0, 24.0));
                     let r = ui.add_enabled(valid, b);
-                    crate::scenario::publish_handle(&format!("dialog:{label}"), r.rect.center());
+                    crate::scenario::publish_handle(&format!("dialog:{raw_label}"), r.rect.center());
                     if r.clicked() {
                         ok = true;
                     }
                 }
-                let close = if matches!(d.kind, Kind::Params { .. } | Kind::Measure { .. }) { "Close" } else { "Cancel" };
+                let raw_close = if matches!(d.kind, Kind::Params { .. } | Kind::Measure { .. }) { "Close" } else { "Cancel" };
+                let close = tr(raw_close);
                 if ui.add(egui::Button::new(close).min_size(vec2(64.0, 24.0))).clicked() {
                     cancel = true;
                 }

crates/ui-egui/src/documents.rs

@@ -307,7 +307,14 @@ pub fn tabs(app: &mut SolveApp, ui: &mut egui::Ui, r: Rect, x0: f32, x1: f32) {
             t.app_bar
         },
     );
-    ui.painter().text(home.center(), Align2::CENTER_CENTER, "Home", FontId::proportional(13.0), if app.home.open { t.text } else { t.app_bar_text });
+    let home_text = app.tr("Home");
+    ui.painter().text(
+        home.center(),
+        Align2::CENTER_CENTER,
+        home_text,
+        FontId::proportional(13.0),
+        if app.home.open { t.text } else { t.app_bar_text },
+    );
     crate::scenario::publish_handle("tab:home", home.center());
     if response.clicked() {
         app.home.show(app.session.revision);
@@ -342,7 +349,8 @@ pub fn tabs(app: &mut SolveApp, ui: &mut egui::Ui, r: Rect, x0: f32, x1: f32) {
         };
         ui.painter().rect_filled(tr, egui::CornerRadius { nw: 5, ne: 5, sw: 0, se: 0 }, fill);
         let ink = if active { t.text } else { t.app_bar_text };
-        let label = format!("{name}{}", if *dirty { " •" } else { "" });
+        let name_display = if name == "Untitled" { app.tr("Untitled") } else { name };
+        let label = format!("{name_display}{}", if *dirty { " •" } else { "" });
         let g = ui.painter().layout_no_wrap(label, FontId::proportional(13.0), ink);
         let text_w = (tr.width() - 30.0).max(10.0);
         let clip = Rect::from_min_size(tr.min, vec2(text_w + 10.0, tr.height()));
@@ -447,23 +455,33 @@ pub fn prompt(app: &mut SolveApp, ctx: &egui::Context) {
     quit_step(app);
     let Some(i) = app.docs.closing else { return };
     let name = tabs_info(app).get(i).map(|x| x.0.clone()).unwrap_or_default();
+    let name_display = if name == "Untitled" { crate::i18n::tr("Untitled") } else { &name };
     let mut answer: Option<Answer> = None;
-    crate::frame::window(ctx, format!("Save changes to {name}?"), crate::frame::Width::Normal)
+    let title = if crate::i18n::current_lang() == "ja" {
+        format!("{name_display} への変更を保存しますか？")
+    } else {
+        format!("Save changes to {name}?")
+    };
+    crate::frame::window(ctx, title, crate::frame::Width::Normal)
         .id(egui::Id::new("sc_close_prompt"))
         .collapsible(false)
         .resizable(false)
         .anchor(Align2::CENTER_CENTER, vec2(0.0, 0.0))
         .show(ctx, |ui| {
-            ui.label(format!("{name} has unsaved changes. If you don't save, they are lost."));
+            if crate::i18n::current_lang() == "ja" {
+                ui.label(format!("{name_display} には保存されていない変更があります。保存しない場合、変更内容は失われます。"));
+            } else {
+                ui.label(format!("{name} has unsaved changes. If you don't save, they are lost."));
+            }
             ui.add_space(8.0);
             ui.horizontal(|ui| {
-                if ui.button("Save").clicked() {
+                if ui.button(crate::i18n::tr("Save")).clicked() {
                     answer = Some(Answer::Save);
                 }
-                if ui.button("Don't Save").clicked() {
+                if ui.button(crate::i18n::tr("Don't Save")).clicked() {
                     answer = Some(Answer::DontSave);
                 }
-                if ui.button("Cancel").clicked() {
+                if ui.button(crate::i18n::tr("Cancel")).clicked() {
                     answer = Some(Answer::Cancel);
                 }
             });

crates/ui-egui/src/fonts.rs

@@ -0,0 +1,46 @@
+//! System font loading and fallback setup for multi-language (CJK/Japanese) support.
+
+use std::sync::Arc;
+
+/// Candidate font paths to look for on different operating systems.
+const FONT_CANDIDATES: &[&str] = &[
+    // Windows
+    "C:\\Windows\\Fonts\\meiryo.ttc",
+    "C:\\Windows\\Fonts\\YuGothM.ttc",
+    "C:\\Windows\\Fonts\\YuGothR.ttc",
+    "C:\\Windows\\Fonts\\msgothic.ttc",
+    "C:\\Windows\\Fonts\\BIZ-UDGothicR.ttc",
+    // macOS
+    "/System/Library/Fonts/Hiragino Sans GB.ttc",
+    "/System/Library/Fonts/PingFang.ttc",
+    "/Library/Fonts/Arial Unicode.ttf",
+    // Linux
+    "/usr/share/fonts/opentype/noto/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/truetype/noto/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/noto-cjk/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/truetype/droid/DroidSansFallbackFull.ttf",
+];
+
+/// Configure font definitions with system CJK fallback if available.
+pub fn setup_fonts(ctx: &egui::Context) {
+    let mut font_bytes: Option<Vec<u8>> = None;
+    for path in FONT_CANDIDATES {
+        if let Ok(bytes) = std::fs::read(path) {
+            font_bytes = Some(bytes);
+            break;
+        }
+    }
+
+    let Some(bytes) = font_bytes else {
+        return;
+    };
+
+    let mut fonts = egui::FontDefinitions::default();
+    fonts.font_data.insert("cjk_fallback".to_owned(), Arc::new(egui::FontData::from_owned(bytes)));
+
+    fonts.families.entry(egui::FontFamily::Proportional).or_default().push("cjk_fallback".to_owned());
+
+    fonts.families.entry(egui::FontFamily::Monospace).or_default().push("cjk_fallback".to_owned());
+
+    ctx.set_fonts(fonts);
+}

crates/ui-egui/src/i18n.rs

@@ -0,0 +1,424 @@
+//! Multi-language (i18n) static translation dictionary for SolveCraft.
+//!
+//! Provides zero-overhead, completely offline local string translations.
+
+/// Supported languages: (id, display_name).
+pub const LANGUAGES: [(&str, &str); 2] = [("en", "English"), ("ja", "日本語")];
+
+/// English to Japanese dictionary.
+const JA_TRANSLATIONS: &[(&str, &str)] = &[
+    // --- Common & Window controls ---
+    ("Preferences", "環境設定"),
+    ("General", "一般"),
+    ("Navigation", "ナビゲーション"),
+    ("Keyboard Shortcuts", "キーボードショートカット"),
+    ("Design", "デザイン"),
+    ("Graphics", "グラフィックス"),
+    ("Sketch", "スケッチ"),
+    ("Selection", "選択"),
+    ("Language", "言語"),
+    ("Theme", "テーマ"),
+    ("UI scale", "UIスケール"),
+    ("Units of new designs", "新規デザインの単位"),
+    ("Autosave every", "自動保存の間隔"),
+    ("Length decimals", "長さの小数点桁数"),
+    ("Angle decimals", "角度の小数点桁数"),
+    ("Agent cursor", "AIエージェントカーソル"),
+    ("Agent cursor speed", "AIエージェント速度"),
+    ("Restore Defaults", "初期値に戻す"),
+    ("Reset the settings on this page", "このページの設定をリセット"),
+    ("OK", "OK"),
+    ("Cancel", "キャンセル"),
+    ("Apply", "適用"),
+    ("Save", "保存"),
+    ("Don't Save", "保存しない"),
+    ("Untitled", "無題"),
+    ("Dark", "ダーク"),
+    ("Light", "ライト"),
+    ("System", "システム"),
+    ("Help", "ヘルプ"),
+    ("About", "SolveCraft について"),
+    ("Close", "閉じる"),
+    ("Delete", "削除"),
+    ("Expand", "展開"),
+    ("Collapse", "折りたたむ"),
+    // --- Preferences Navigation & Design ---
+    ("Pan, zoom, orbit", "パン、ズーム、オービット"),
+    ("Reverse zoom direction", "ズーム方向を反転"),
+    ("Zoom to the cursor", "カーソル位置へズーム"),
+    ("Orbit around", "回転の中心"),
+    ("View centre", "ビュー中心"),
+    ("Point under the cursor", "カーソル下の点"),
+    ("Animate view changes", "視点変更をアニメーション表示"),
+    ("Animation speed", "アニメーション速度"),
+    ("Default modelling orientation", "デフォルトのモデル方向"),
+    ("Offer to stand up Y-up files", "Y-upファイルを立ち上げるか確認"),
+    ("Visual style", "表示スタイル"),
+    // --- Application bar & Menus ---
+    ("File", "ファイル"),
+    ("Edit", "編集"),
+    ("Create", "作成"),
+    ("Modify", "修正"),
+    ("Assemble", "アセンブリ"),
+    ("Construct", "構築"),
+    ("Inspect", "検査"),
+    ("Insert", "挿入"),
+    ("Select", "選択"),
+    ("New Design", "新規デザイン"),
+    ("Open...", "開く..."),
+    ("Open", "開く"),
+    ("Save", "保存"),
+    ("Save As...", "名前を付けて保存..."),
+    ("Export...", "エクスポート..."),
+    ("Undo", "元に戻す"),
+    ("Redo", "やり直す"),
+    ("Cut", "切り取り"),
+    ("Copy", "コピー"),
+    ("Paste", "貼り付け"),
+    ("Clear Recent", "最近使った項目をクリア"),
+    ("Open Recent", "最近使った項目を開く"),
+    ("Quit", "終了"),
+    // --- Toolbar tabs ---
+    ("SOLID", "ソリッド"),
+    ("SURFACE", "サーフェス"),
+    ("MESH", "メッシュ"),
+    ("SHEET METAL", "板金"),
+    ("PLASTIC", "プラスチック"),
+    ("SKETCH", "スケッチ"),
+    ("CREATE", "作成"),
+    ("MODIFY", "修正"),
+    ("ASSEMBLE", "アセンブリ"),
+    ("CONSTRUCT", "構築"),
+    ("INSPECT", "検査"),
+    ("INSERT", "挿入"),
+    ("SELECT", "選択"),
+    ("FINISH SKETCH", "スケッチを終了"),
+    ("Finish Sketch", "スケッチを終了"),
+    // --- Sketch Palette ---
+    ("SKETCH PALETTE", "スケッチ パレット"),
+    ("Options", "オプション"),
+    ("Linetype", "線種"),
+    ("Normal", "標準"),
+    ("Construction", "構築"),
+    ("Centerline", "中心線"),
+    ("Look At", "視点を合わせる"),
+    ("Sketch Grid", "スケッチ グリッド"),
+    ("Snap", "スナップ"),
+    ("Slice", "スライス"),
+    ("Show Profile", "プロファイルを表示"),
+    ("Show Points", "点を表示"),
+    ("Show Dimensions", "寸法を表示"),
+    ("Show Constraints", "拘束を表示"),
+    ("Show Projected Geometries", "投影ジオメトリを表示"),
+    ("3D Sketch", "3D スケッチ"),
+    ("Fully constrained", "完全拘束"),
+    ("1 degree of freedom", "自由度 1"),
+    ("degrees of freedom", "自由度"),
+    ("Constraints conflict", "拘束が競合しています"),
+    // --- Dialog Titles (All Caps & Title Case) ---
+    ("CREATE SKETCH", "スケッチを作成"),
+    ("Create Sketch", "スケッチを作成"),
+    ("EDIT", "編集"),
+    ("EXTRUDE", "押し出し"),
+    ("Extrude", "押し出し"),
+    ("REVOLVE", "回転"),
+    ("Revolve", "回転"),
+    ("SWEEP", "スイープ"),
+    ("Sweep", "スイープ"),
+    ("LOFT", "ロフト"),
+    ("Loft", "ロフト"),
+    ("FILLET", "フィレット"),
+    ("Fillet", "フィレット"),
+    ("CHAMFER", "面取り"),
+    ("Chamfer", "面取り"),
+    ("SHELL", "シェル"),
+    ("Shell", "シェル"),
+    ("DRAFT", "勾配"),
+    ("Draft", "勾配"),
+    ("SCALE", "尺度"),
+    ("Scale", "尺度"),
+    ("COMBINE", "結合"),
+    ("Combine", "結合"),
+    ("SPLIT BODY", "ボディを分割"),
+    ("Split Body", "ボディを分割"),
+    ("MIRROR", "ミラー"),
+    ("Mirror", "ミラー"),
+    ("HOLE", "穴"),
+    ("Hole", "穴"),
+    ("THREAD", "ねじ"),
+    ("Thread", "ねじ"),
+    ("BOX", "直方体"),
+    ("Box", "直方体"),
+    ("CYLINDER", "円柱"),
+    ("Cylinder", "円柱"),
+    ("SPHERE", "球"),
+    ("Sphere", "球"),
+    ("TORUS", "トーラス"),
+    ("Torus", "トーラス"),
+    ("COIL", "コイル"),
+    ("Coil", "コイル"),
+    ("PIPE", "パイプ"),
+    ("Pipe", "パイプ"),
+    ("OFFSET PLANE", "オフセット平面"),
+    ("Offset Plane", "オフセット平面"),
+    ("PLANE AT ANGLE", "傾斜平面"),
+    ("Plane at Angle", "傾斜平面"),
+    ("MEASURE", "計測"),
+    ("Measure", "計測"),
+    ("SECTION ANALYSIS", "断面解析"),
+    ("Section Analysis", "断面解析"),
+    ("PARAMETERS", "パラメータ"),
+    ("Parameters", "パラメータ"),
+    ("MOVE", "移動"),
+    ("Move", "移動"),
+    ("RECTANGULAR PATTERN", "矩形パターン"),
+    ("Rectangular Pattern", "矩形パターン"),
+    ("CIRCULAR PATTERN", "円形パターン"),
+    ("Circular Pattern", "円形パターン"),
+    ("PATTERN ON PATH", "パス上のパターン"),
+    ("Pattern on Path", "パス上のパターン"),
+    // --- Dialog Row Labels and Inputs ---
+    ("Plane", "平面"),
+    ("Profile", "プロファイル"),
+    ("Profiles", "プロファイル"),
+    ("Axis", "軸"),
+    ("Axes", "軸"),
+    ("Path", "パス"),
+    ("Points", "点"),
+    ("Objects", "オブジェクト"),
+    ("Bodies", "ボディ"),
+    ("Features", "フィーチャー"),
+    ("Start", "開始"),
+    ("Offset", "オフセット"),
+    ("Profile Plane", "プロファイル平面"),
+    ("Direction", "方向"),
+    ("One Side", "片側"),
+    ("Two Sides", "両側"),
+    ("Symmetric", "対称"),
+    ("Extent Type", "範囲タイプ"),
+    ("Distance", "距離"),
+    ("Distance 1", "距離 1"),
+    ("Distance 2", "距離 2"),
+    ("To Object", "オブジェクトまで"),
+    ("All", "すべて"),
+    ("Taper Angle", "テーパ角度"),
+    ("Operation", "操作"),
+    ("New Body", "新規ボディ"),
+    ("Join", "結合"),
+    ("Cut", "切り取り"),
+    ("Intersect", "交差"),
+    ("Angle", "角度"),
+    ("Radius", "半径"),
+    ("Radius Type", "半径タイプ"),
+    ("Constant", "一定"),
+    ("Chord Length", "弦長"),
+    ("Variable", "可変"),
+    ("Start Radius", "開始半径"),
+    ("End Radius", "終了半径"),
+    ("Flip", "反転"),
+    ("Thickness", "厚さ"),
+    ("Placement", "配置"),
+    ("Single", "単一"),
+    ("Multiple", "複数"),
+    ("Hole Type", "穴タイプ"),
+    ("Simple", "単純"),
+    ("Counterbore", "座ぐり"),
+    ("Countersink", "皿もみ"),
+    ("Hole Tap Type", "タップタイプ"),
+    ("Tapped", "タップ立て"),
+    ("Drill Point", "先端"),
+    ("Flat", "フラット"),
+    ("Tip Angle", "先端角度"),
+    ("Diameter", "直径"),
+    ("Depth", "深さ"),
+    ("Object Type", "オブジェクトタイプ"),
+    ("Clear the selection", "選択をクリア"),
+    // --- Browser Tree ---
+    ("Document Settings", "ドキュメントの設定"),
+    ("Units", "単位"),
+    ("Named Views", "名前付きビュー"),
+    ("Origin", "原点"),
+    ("Top", "上面"),
+    ("Front", "正面"),
+    ("Right", "右側面"),
+    ("Home", "ホーム"),
+    // --- Sketch commands & tools ---
+    ("Line", "線分"),
+    ("Rectangle", "長方形"),
+    ("Circle", "円"),
+    ("Arc", "円弧"),
+    ("Polygon", "ポリゴン"),
+    ("Ellipse", "楕円"),
+    ("Slot", "スロット"),
+    ("Spline", "スプライン"),
+    ("Conic Curve", "円錐曲線"),
+    ("Point", "点"),
+    ("Text", "テキスト"),
+    ("Trim", "トリム"),
+    ("Extend", "延長"),
+    ("Break", "分割"),
+    ("Sketch Scale", "スケッチ尺度"),
+    ("Project", "投影"),
+    ("Dimension", "寸法"),
+    ("Sketch Dimension", "スケッチ寸法"),
+    ("Horizontal/Vertical", "水平/垂直"),
+    ("Coincident", "一致"),
+    ("Tangent", "正接"),
+    ("Equal", "等しい"),
+    ("Parallel", "平行"),
+    ("Perpendicular", "直交"),
+    ("Fix/Unfix", "固定/固定解除"),
+    ("Midpoint", "中点"),
+    ("Concentric", "同心円"),
+    ("Collinear", "同一直線"),
+    ("Symmetry", "対称"),
+    ("Curvature", "曲率"),
+    ("Smooth", "スムーズ"),
+    // --- Workspace selector & environments ---
+    ("DESIGN", "デザイン"),
+    ("workspace", "ワークスペース"),
+    ("Change workspace", "ワークスペースの変更"),
+    ("Generative Design", "ジェネレーティブ デザイン"),
+    ("Render", "レンダリング"),
+    ("Animation", "アニメーション"),
+    ("Simulation", "シミュレーション"),
+    ("Manufacture", "製造"),
+    ("Drawing", "図面"),
+    ("coming later", "準備中"),
+    ("current", "現在"),
+    // --- Browser Tree ---
+    ("Document Settings", "ドキュメントの設定"),
+    ("Units", "単位"),
+    ("Named Views", "名前付きビュー"),
+    ("Origin", "原点"),
+    ("Top", "上面"),
+    ("Front", "正面"),
+    ("Right", "右側面"),
+    ("Home", "ホーム"),
+    ("TOP", "上面"),
+    ("FRONT", "正面"),
+    ("RIGHT", "右側面"),
+    ("HOME", "ホーム"),
+    ("Bodies", "ボディ"),
+    ("Sketches", "スケッチ"),
+    ("Canvases", "キャンバス"),
+    ("Joints", "ジョイント"),
+    ("Construction", "構築"),
+    ("Collapse", "折りたたむ"),
+    ("Expand", "展開"),
+    ("Hide", "非表示"),
+    ("Show", "表示"),
+    ("Active component", "アクティブなコンポーネント"),
+    ("Activate", "アクティブ化"),
+];
+
+thread_local! {
+    static CURRENT_LANG: std::cell::RefCell<String> = const { std::cell::RefCell::new(String::new()) };
+}
+
+/// Sets the current UI language for the thread.
+pub fn set_current_lang(lang: &str) {
+    CURRENT_LANG.with(|l| {
+        *l.borrow_mut() = lang.to_string();
+    });
+}
+
+/// Gets the current UI language for the thread.
+pub fn current_lang() -> String {
+    CURRENT_LANG.with(|l| l.borrow().clone())
+}
+
+/// Translate text using the thread's current language.
+pub fn tr(text: &str) -> &str {
+    CURRENT_LANG.with(|l| {
+        let lang = l.borrow();
+        if lang.as_str() == "ja" { tr_for_lang("ja", text) } else { text }
+    })
+}
+
+/// Translate text using a specified language.
+pub fn tr_for_lang<'a>(lang: &str, text: &'a str) -> &'a str {
+    if lang == "ja" {
+        for &(en, ja) in JA_TRANSLATIONS {
+            if en == text {
+                return ja;
+            }
+        }
+    }
+    text
+}
+
+/// Dynamically translate labels including patterned names like "Sketch1", "Body2", "Units: mm", etc.
+pub fn tr_dyn(text: &str) -> String {
+    CURRENT_LANG.with(|l| {
+        let lang = l.borrow();
+        if lang.as_str() != "ja" {
+            return text.to_string();
+        }
+        for &(en, ja) in JA_TRANSLATIONS {
+            if en == text {
+                return ja.to_string();
+            }
+        }
+        if let Some(rest) = text.strip_prefix("Sketch")
+            && !rest.is_empty()
+            && rest.chars().all(|c| c.is_ascii_digit())
+        {
+            return format!("スケッチ{rest}");
+        }
+        if let Some(rest) = text.strip_prefix("Body")
+            && !rest.is_empty()
+            && rest.chars().all(|c| c.is_ascii_digit())
+        {
+            return format!("ボディ{rest}");
+        }
+        if let Some(rest) = text.strip_prefix("Component")
+            && !rest.is_empty()
+            && rest.chars().all(|c| c.is_ascii_digit())
+        {
+            return format!("コンポーネント{rest}");
+        }
+        if let Some(rest) = text.strip_prefix("Units: ") {
+            return format!("単位: {rest}");
+        }
+        if let Some(rest) = text.strip_prefix("Precision: ") {
+            return format!("精度: {rest}");
+        }
+        if let Some(rest) = text.strip_prefix("Parameters (") {
+            return format!("パラメータ ({rest}");
+        }
+        text.to_string()
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn translates_japanese() {
+        set_current_lang("ja");
+        assert_eq!(tr("Preferences"), "環境設定");
+        assert_eq!(tr("Extrude"), "押し出し");
+        assert_eq!(tr("CREATE SKETCH"), "スケッチを作成");
+        assert_eq!(tr("SKETCH PALETTE"), "スケッチ パレット");
+        assert_eq!(tr("Finish Sketch"), "スケッチを終了");
+        assert_eq!(tr("FINISH SKETCH"), "スケッチを終了");
+        assert_eq!(tr("Generative Design"), "ジェネレーティブ デザイン");
+        assert_eq!(tr("coming later"), "準備中");
+        assert_eq!(tr("Sketches"), "スケッチ");
+        assert_eq!(tr("NonExistentKey"), "NonExistentKey");
+
+        assert_eq!(tr_dyn("Sketch1"), "スケッチ1");
+        assert_eq!(tr_dyn("Body2"), "ボディ2");
+        assert_eq!(tr_dyn("Units: mm"), "単位: mm");
+    }
+
+    #[test]
+    fn keeps_english() {
+        set_current_lang("en");
+        assert_eq!(tr("Preferences"), "Preferences");
+        assert_eq!(tr("Extrude"), "Extrude");
+        assert_eq!(tr_dyn("Sketch1"), "Sketch1");
+    }
+}

crates/ui-egui/src/lib.rs

@@ -27,11 +27,13 @@ pub mod dialogs_surface;
 pub mod dim_view;
 pub mod documents;
 pub mod drag_snap;
+pub mod fonts;
 pub mod frame;
 pub mod gizmo;
 pub mod gpu;
 pub mod help;
 pub mod home;
+pub mod i18n;
 pub mod icons;
 pub mod inference;
 pub mod keymap;
@@ -288,6 +290,11 @@ impl SolveApp {
         }
     }
 
+    /// Translate text using the active language preference.
+    pub fn tr<'a>(&self, text: &'a str) -> &'a str {
+        i18n::tr_for_lang(&self.preferences.language, text)
+    }
+
     /// Draw the viewport on the GPU (eframe's wgpu render state, with the app's depth and
     /// stencil buffer bits and MSAA sample count). Without it the viewport renders on the CPU.
     pub fn set_wgpu(&mut self, rs: &egui_wgpu::RenderState, depth_bits: u8, stencil_bits: u8, samples: u32) {
@@ -813,6 +820,7 @@ impl SolveApp {
     /// Lay out the whole window.
     pub fn ui(&mut self, ui: &mut egui::Ui) {
         let t0 = now_ms();
+        crate::i18n::set_current_lang(&self.preferences.language);
         scenario::clear_handles();
         toolbar::app_bar(self, ui);
         file_load::show(self, ui.ctx());

crates/ui-egui/src/prefs.rs

@@ -46,6 +46,8 @@ pub struct Prefs {
     pub angle_decimals: u8,
     /// `dark`, `light` or `system` (follow the operating system).
     pub theme: String,
+    /// Interface language (`en`, `ja`).
+    pub language: String,
     /// Interface scale (1 = 100 %).
     pub ui_scale: f32,
     /// The wheel zooms toward the point under the cursor (else toward the view's centre).
@@ -70,6 +72,7 @@ impl Default for Prefs {
             length_decimals: 2,
             angle_decimals: 1,
             theme: "dark".into(),
+            language: "en".into(),
             ui_scale: 1.0,
             zoom_to_cursor: true,
             look_at_anim: true,
@@ -157,6 +160,7 @@ fn defaults(app: &mut SolveApp, page: usize) {
     match page {
         0 => {
             p.theme = d.theme;
+            p.language = d.language;
             p.ui_scale = d.ui_scale;
             p.default_units = d.default_units;
             p.length_decimals = d.length_decimals;
@@ -376,7 +380,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     let mut open = true;
     let mut button: Option<&str> = None;
     let before = app.preferences.clone();
-    crate::frame::window(ctx, "Preferences", crate::frame::Width::Wide)
+    crate::frame::window(ctx, app.tr("Preferences"), crate::frame::Width::Wide)
         .id(egui::Id::new("sc_prefs"))
         .open(&mut open)
         .collapsible(false)
@@ -389,7 +393,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                 ui.vertical(|ui| {
                     ui.set_width(140.0);
                     for (i, s) in PAGES.iter().enumerate() {
-                        if ui.selectable_label(app.prefs_window.section == i, *s).clicked() {
+                        if ui.selectable_label(app.prefs_window.section == i, app.tr(s)).clicked() {
                             app.prefs_window.section = i;
                             app.keymap.read_only = false;
                         }
@@ -399,24 +403,24 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                 ui.vertical(|ui| {
                     // A fixed width, so the window keeps its size from page to page.
                     ui.set_width(540.0);
-                    ui.label(RichText::new(PAGES.get(app.prefs_window.section).copied().unwrap_or_default()).strong().size(14.0));
+                    ui.label(RichText::new(app.tr(PAGES.get(app.prefs_window.section).copied().unwrap_or_default())).strong().size(14.0));
                     ui.add_space(6.0);
                     page(app, ui, &t);
                 });
             });
             ui.separator();
             ui.horizontal(|ui| {
-                if ui.button("Restore Defaults").on_hover_text("Reset the settings on this page").clicked() {
+                if ui.button(app.tr("Restore Defaults")).on_hover_text(app.tr("Reset the settings on this page")).clicked() {
                     button = Some("defaults");
                 }
                 crate::frame::right_aligned(ui, 700.0, 170.0, |ui| {
-                    if ui.button("OK").clicked() {
+                    if ui.button(app.tr("OK")).clicked() {
                         button = Some("ok");
                     }
-                    if ui.button("Cancel").clicked() {
+                    if ui.button(app.tr("Cancel")).clicked() {
                         button = Some("cancel");
                     }
-                    if ui.button("Apply").clicked() {
+                    if ui.button(app.tr("Apply")).clicked() {
                         button = Some("apply");
                     }
                 });
@@ -478,10 +482,19 @@ fn page(app: &mut SolveApp, ui: &mut egui::Ui, t: &Tokens) {
     }
     egui::Grid::new(("sc_prefs_grid", app.prefs_window.section)).num_columns(2).spacing([16.0, 8.0]).show(ui, |ui| match app.prefs_window.section {
         0 => {
-            ui.label("Theme");
+            ui.label(app.tr("Language"));
+            egui::ComboBox::from_id_salt("pref_language")
+                .selected_text(crate::i18n::LANGUAGES.iter().find(|(id, _)| *id == app.preferences.language).map_or("English", |x| x.1))
+                .show_ui(ui, |ui| {
+                    for (id, label) in crate::i18n::LANGUAGES {
+                        ui.selectable_value(&mut app.preferences.language, id.to_string(), label);
+                    }
+                });
+            ui.end_row();
+            ui.label(app.tr("Theme"));
             ui.horizontal(|ui| {
                 for (id, label) in THEMES {
-                    if ui.radio(app.preferences.theme == id, label).clicked() {
+                    if ui.radio(app.preferences.theme == id, app.tr(label)).clicked() {
                         app.preferences.theme = id.to_string();
                         if id != "system" {
                             app.ui.dark = id == "dark";
@@ -490,33 +503,34 @@ fn page(app: &mut SolveApp, ui: &mut egui::Ui, t: &Tokens) {
                 }
             });
             ui.end_row();
-            ui.label("UI scale");
+            ui.label(app.tr("UI scale"));
             ui.add(egui::Slider::new(&mut app.preferences.ui_scale, 0.75..=2.0).step_by(0.05).custom_formatter(|v, _| format!("{:.0} %", v * 100.0)));
             ui.end_row();
-            ui.label("Units of new designs");
+            ui.label(app.tr("Units of new designs"));
             egui::ComboBox::from_id_salt("pref_units").selected_text(app.preferences.default_units.clone()).show_ui(ui, |ui| {
                 for u in UNITS {
                     ui.selectable_value(&mut app.preferences.default_units, u.to_string(), u);
                 }
             });
             ui.end_row();
-            ui.label("Autosave every");
+            ui.label(app.tr("Autosave every"));
             if ui.add(egui::DragValue::new(&mut app.autosave_minutes).range(0.0..=120.0).speed(0.5).suffix(" min")).changed() {
                 set_autosave(app);
             }
             ui.end_row();
             note(ui, "0 turns it off");
-            ui.label("Length decimals");
+            ui.label(app.tr("Length decimals"));
             ui.add(egui::DragValue::new(&mut app.preferences.length_decimals).range(0..=6));
             ui.end_row();
-            ui.label("Angle decimals");
+            ui.label(app.tr("Angle decimals"));
             ui.add(egui::DragValue::new(&mut app.preferences.angle_decimals).range(0..=6));
             ui.end_row();
-            ui.label("Agent cursor");
+            ui.label(app.tr("Agent cursor"));
+            let speed_label = app.tr("Agent cursor speed");
             let s = &mut app.ui.agent_cursor;
             ui.checkbox(&mut s.show, "Show the pointer an AI agent moves (#35)");
             ui.end_row();
-            ui.label("Agent cursor speed");
+            ui.label(speed_label);
             ui.add_enabled_ui(s.show, |ui| {
                 crate::agent_cursor::speed_slider(ui, &mut s.speed);
             });
@@ -693,6 +707,9 @@ pub fn set(app: &mut SolveApp, p: &serde_json::Value) -> Result<serde_json::Valu
     if !THEMES.iter().any(|(id, _)| *id == new.theme) || !["z", "y"].contains(&new.up_axis.as_str()) {
         return Err("`theme` must be dark, light or system and `up_axis` z or y".into());
     }
+    if !crate::i18n::LANGUAGES.iter().any(|(id, _)| *id == new.language) {
+        return Err("`language` must be en or ja".into());
+    }
     if !(0.5..=2.5).contains(&new.ui_scale) || !(0.25..=4.0).contains(&new.look_at_speed) {
         return Err("`ui_scale` must be 0.5…2.5 and `look_at_speed` 0.25…4".into());
     }
@@ -778,11 +795,33 @@ mod tests {
         // File and Help › Keyboard Shortcuts open the same dialog at its page.
         app.keymap.open = true;
         let ctx = egui::Context::default();
-        let _ = ctx.run_ui(Default::default(), |ui| show(&mut app, ui.ctx()));
+        let mut out = ctx.run_ui(Default::default(), |ui| show(&mut app, ui.ctx()));
+        out.textures_delta.clear();
         assert!(app.prefs_window.open && app.prefs_window.section == PAGE_SHORTCUTS && !app.keymap.open);
         assert!(control(&mut app, &json!({"open": "Plugins"})).is_err());
         assert!(set(&mut app, &json!({"up_axis": "x"})).is_err());
         assert!(set(&mut app, &json!({"theme": "sepia"})).is_err());
+        assert!(set(&mut app, &json!({"language": "fr"})).is_err());
+    }
+
+    #[test]
+    fn language_preference_switches_and_persists() {
+        let mut app = SolveApp::new(solvecraft_engine::Session::default(), Default::default());
+        assert_eq!(app.preferences.language, "en");
+        assert_eq!(app.tr("Preferences"), "Preferences");
+        assert_eq!(app.tr("Extrude"), "Extrude");
+
+        // Switch language to Japanese
+        set(&mut app, &json!({"language": "ja"})).unwrap();
+        assert_eq!(app.preferences.language, "ja");
+        assert_eq!(app.tr("Preferences"), "環境設定");
+        assert_eq!(app.tr("Extrude"), "押し出し");
+
+        // Preferences round-trip
+        let mut other = SolveApp::new(solvecraft_engine::Session::default(), Default::default());
+        other.load_prefs(&app.prefs());
+        assert_eq!(other.preferences.language, "ja");
+        assert_eq!(other.tr("Preferences"), "環境設定");
     }
 
     /// A STEP file opened Z-up is offered to be stood up: a quarter turn about X, undoable.

crates/ui-egui/src/sketch_palette.rs

@@ -201,7 +201,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     let mut finish = false;
     let has_curves = !selected_curves(app).is_empty();
     crate::frame::docked(
-        "SKETCH PALETTE",
+        app.tr("SKETCH PALETTE"),
         egui::Id::new("sc_sketch_palette"),
         anchor,
         ctx.content_rect().bottom() - anchor.y - 8.0,
@@ -215,39 +215,47 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
             if ui.add(egui::Button::new(RichText::new(if folded { "+" } else { "−" }).size(14.0)).frame(false)).clicked() {
                 FOLDED.with(|f| f.set(!folded));
             }
-            ui.label(RichText::new("SKETCH PALETTE").strong().size(12.5));
+            ui.label(RichText::new(app.tr("SKETCH PALETTE")).strong().size(12.5));
         });
         if FOLDED.with(Cell::get) {
             return;
         }
         let ofold = OPTIONS_FOLDED.with(Cell::get);
-        if ui.add(egui::Button::new(RichText::new(format!("{} Options", if ofold { "+" } else { "−" })).size(12.0)).frame(false)).clicked() {
+        if ui
+            .add(egui::Button::new(RichText::new(format!("{} {}", if ofold { "+" } else { "−" }, app.tr("Options"))).size(12.0)).frame(false))
+            .clicked()
+        {
             OPTIONS_FOLDED.with(|f| f.set(!ofold));
         }
         if !ofold {
             egui::Grid::new("sc_palette_grid").num_columns(2).spacing(vec2(10.0, 6.0)).show(ui, |ui| {
-                ui.label("Linetype");
+                ui.label(app.tr("Linetype"));
                 ui.horizontal(|ui| {
                     // With a selection: convert it; without: the mode new curves are drawn in.
                     let current = mode(app);
                     for (i, (label, tip)) in [("—", "Normal"), ("- -", "Construction"), ("-·-", "Centerline")].iter().enumerate() {
                         let on = !has_curves && current == i as u8;
-                        let tip = if has_curves { format!("Make the selection {tip}") } else { format!("Draw {tip} curves") };
+                        let tr_tip = app.tr(tip);
-                        if ui.add(egui::Button::new(*label).selected(on).min_size(vec2(30.0, 20.0))).on_hover_text(tip).clicked() {
+                        let tip_str = if has_curves {
+                            format!("{} ({})", app.tr("Make the selection"), tr_tip)
+                        } else {
+                            format!("{} ({})", app.tr("Draw curves"), tr_tip)
+                        };
+                        if ui.add(egui::Button::new(*label).selected(on).min_size(vec2(30.0, 20.0))).on_hover_text(tip_str).clicked() {
                             linetype = Some(i as u8);
                         }
                     }
                 });
                 ui.end_row();
-                ui.label("Look At");
+                ui.label(app.tr("Look At"));
-                if ui.button("Look At").clicked() {
+                if ui.button(app.tr("Look At")).clicked() {
                     look = true;
                 }
                 ui.end_row();
                 for (k, label) in TOGGLES {
                     // The label is part of the checkbox: clicking it toggles too.
                     let mut on = shown(k);
-                    if ui.checkbox(&mut on, label).changed() {
+                    if ui.checkbox(&mut on, app.tr(label)).changed() {
                         set = Some((k.to_string(), on));
                     }
                     ui.end_row();
@@ -255,11 +263,12 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
             });
         }
         ui.add_space(4.0);
-        ui.label(RichText::new(dof_line.as_str()).color(if ok { t.sketch_accent } else { t.text_dim }));
+        let tr_dof = app.tr(&dof_line);
+        ui.label(RichText::new(tr_dof).color(if ok { t.sketch_accent } else { t.text_dim }));
         ui.add_space(4.0);
         ui.horizontal(|ui| {
             ui.add_space(WIDTH - 100.0);
-            if ui.add(egui::Button::new(RichText::new("Finish Sketch").color(Color32::WHITE)).fill(t.accent)).clicked() {
+            if ui.add(egui::Button::new(RichText::new(app.tr("Finish Sketch")).color(Color32::WHITE)).fill(t.accent)).clicked() {
                 finish = true;
             }
         });

crates/ui-egui/src/toolbar.rs

@@ -68,26 +68,26 @@ pub fn app_bar(app: &mut SolveApp, ui: &mut egui::Ui) {
             );
             resp.on_hover_text(tip).clicked()
         };
-        let file = click(ui, "folder", "File");
+        let file = click(ui, "folder", app.tr("File"));
-        if click(ui, "save", "Save (Ctrl+S)") {
+        if click(ui, "save", &format!("{} (Ctrl+S)", app.tr("Save"))) {
             save(app);
         }
-        if click(ui, "undo", "Undo (Ctrl+Z)") {
+        if click(ui, "undo", &format!("{} (Ctrl+Z)", app.tr("Undo"))) {
             let _ = app.run("edit.undo", json!({}));
         }
-        if click(ui, "redo", "Redo (Ctrl+Y)") {
+        if click(ui, "redo", &format!("{} (Ctrl+Y)", app.tr("Redo"))) {
             let _ = app.run("edit.redo", json!({}));
         }
-        if click(ui, "home", "Start page") {
+        if click(ui, "home", app.tr("Home")) {
             let rev = app.session.revision;
             app.home.show(rev);
         }
-        let theme_tip = if app.ui.dark { "Light theme" } else { "Dark theme" };
+        let theme_tip = if app.ui.dark { app.tr("Light") } else { app.tr("Dark") };
         if click(ui, if app.ui.dark { "sun" } else { "moon" }, theme_tip) {
             app.ui.dark = !app.ui.dark;
             app.preferences.theme = if app.ui.dark { "dark" } else { "light" }.into();
         }
-        if click(ui, "help", "Help") {
+        if click(ui, "help", app.tr("Help")) {
             app.help.menu = !app.help.menu;
             let at = ui.ctx().input(|i| i.pointer.latest_pos()).map_or(300.0, |p| p.x);
             ui.ctx().data_mut(|d| {
@@ -118,7 +118,9 @@ pub fn app_bar(app: &mut SolveApp, ui: &mut egui::Ui) {
         }
         icons::paint(ui.painter(), gear.shrink(4.0), "settings", t.app_bar_text, t.app_bar_text, t.app_bar);
         crate::scenario::publish_handle("titlebar:preferences", gear.center());
-        if gresp.on_hover_text(if cfg!(target_os = "macos") { "Preferences (Cmd+,)" } else { "Preferences (Ctrl+,)" }).clicked() {
+        let prefs_tip =
+            if cfg!(target_os = "macos") { format!("{} (Cmd+,)", app.tr("Preferences")) } else { format!("{} (Ctrl+,)", app.tr("Preferences")) };
+        if gresp.on_hover_text(prefs_tip).clicked() {
             crate::prefs::open_at(app, 0);
         }
         ui.painter().text(
@@ -155,17 +157,17 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                 let item = |ui: &mut egui::Ui, label: &str, key: &str| {
                     ui.add(egui::Button::new(label).shortcut_text(key).min_size(vec2(180.0, 22.0))).clicked()
                 };
-                if item(ui, "New Design", "Ctrl+N") {
+                if item(ui, app.tr("New Design"), "Ctrl+N") {
                     crate::documents::new_design(app);
                     close = true;
                 }
-                if item(ui, "Open…", "Ctrl+O") {
+                if item(ui, &format!("{}…", app.tr("Open")), "Ctrl+O") {
                     if let Some(p) = app.services.pick_open.as_ref().and_then(|f| f()) {
                         app.open_path_async(&p);
                     }
                     close = true;
                 }
-                ui.menu_button("Open Recent", |ui| {
+                ui.menu_button(app.tr("Open Recent"), |ui| {
                     for path in app.home.recent.clone() {
                         let missing = !solvecraft_engine::io::vfs::exists(&path);
                         let label = format!("{path}{}", if missing { " (missing)" } else { "" });
@@ -176,7 +178,7 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                         }
                     }
                     ui.separator();
-                    if ui.button("Clear Recent").clicked() {
+                    if ui.button(app.tr("Clear Recent")).clicked() {
                         app.home.recent.clear();
                         app.home.opened_at.clear();
                         close = true;
@@ -191,11 +193,11 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                     app.start("file.insert_mesh");
                     close = true;
                 }
-                if item(ui, "Save", "Ctrl+S") {
+                if item(ui, app.tr("Save"), "Ctrl+S") {
                     save(app);
                     close = true;
                 }
-                if item(ui, "Save As…", "") {
+                if item(ui, &format!("{}…", app.tr("Save As")), "") {
                     save_as(app);
                     close = true;
                 }
@@ -217,16 +219,16 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                     close = true;
                 }
                 ui.separator();
-                if item(ui, "Preferences…", "") {
+                if item(ui, &format!("{}…", app.tr("Preferences")), "") {
                     app.prefs_window.open = true;
                     close = true;
                 }
-                if item(ui, "Keyboard Shortcuts…", "") {
+                if item(ui, &format!("{}…", app.tr("Keyboard Shortcuts")), "") {
                     app.keymap.open = true;
                     app.keymap.read_only = false;
                     close = true;
                 }
-                if item(ui, "Quit", "") {
+                if item(ui, app.tr("Quit"), "") {
                     crate::documents::request_quit(app);
                     close = true;
                 }
@@ -282,13 +284,13 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
         painter.text(
             pos2(ws.center().x - 5.0, ws.top() + 52.0),
             Align2::CENTER_CENTER,
-            crate::toolbar_custom::WORKSPACE,
+            app.tr(crate::toolbar_custom::WORKSPACE),
             FontId::proportional(12.0),
             t.text,
         );
         caret(&painter, pos2(ws.center().x + 27.0, ws.top() + 52.0), t.text);
-        painter.text(pos2(ws.center().x, ws.top() + 69.0), Align2::CENTER_CENTER, "workspace", FontId::proportional(10.0), t.text_dim);
+        painter.text(pos2(ws.center().x, ws.top() + 69.0), Align2::CENTER_CENTER, app.tr("workspace"), FontId::proportional(10.0), t.text_dim);
-        if wresp.on_hover_text("Change workspace").clicked() {
+        if wresp.on_hover_text(app.tr("Change workspace")).clicked() {
             crate::context_menu::open_for(app, ws.left_bottom(), crate::context_menu::Target::Workspace);
         }
         crate::scenario::publish_rect("toolbar", r);
@@ -300,7 +302,8 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
         }
         tabs.extend(TABS.iter());
         for tab in tabs {
-            let w = 12.0 + tab.len() as f32 * 7.4;
+            let display_tab = app.tr(tab);
+            let w = 12.0 + (display_tab.len() as f32 * 7.4).max(28.0);
             let tr = Rect::from_min_size(pos2(x, tabs_y), vec2(w, 22.0));
             let active = app.ui.tab == tab;
             let resp = ui.interact(tr, ui.id().with(("tab", tab)), Sense::click());
@@ -317,7 +320,7 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
             } else {
                 t.text_dim
             };
-            painter.text(tr.center(), Align2::CENTER_CENTER, tab, FontId::proportional(12.0), col);
+            painter.text(tr.center(), Align2::CENTER_CENTER, display_tab, FontId::proportional(12.0), col);
             if resp.clicked() {
                 app.ui.tab = tab.to_string();
             }
@@ -384,9 +387,10 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
                 };
                 let icon = if c.id == "sketch.finish" { "finish" } else { c.icon };
                 icons::paint(&painter, br.shrink(6.0), icon, ink, fill, acc);
+                let tr_label = app.tr(c.label);
                 let tip = match &info.disabled_reason {
-                    Some(why) => format!("{}\n{why}", c.label),
+                    Some(why) => format!("{}\n{why}", tr_label),
-                    None => format!("{}{}", c.label, c.shortcut.map(|k| format!("  ({k})")).unwrap_or_default()),
+                    None => format!("{}{}", tr_label, c.shortcut.map(|k| format!("  ({k})")).unwrap_or_default()),
                 };
                 if resp.on_hover_text(tip).clicked() && info.enabled {
                     let id = c.id;
@@ -399,7 +403,7 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
             }
             // Panel label with drop-down.
             let lr = Rect::from_min_size(pos2(px, top + 42.0), vec2(width, 18.0));
-            let label = panel.to_string();
+            let label = app.tr(panel).to_string();
             let lresp = ui.interact(lr, ui.id().with(("panel", panel)), Sense::click());
             crate::scenario::publish_handle(&format!("panel:{tab}:{panel}"), lr.center());
             if lresp.hovered() && enabled_panel {
@@ -437,7 +441,10 @@ pub fn toolbar(app: &mut SolveApp, ui: &mut egui::Ui) {
                             ui.horizontal(|ui| {
                                 let (ir, _) = ui.allocate_exact_size(vec2(18.0, 18.0), Sense::hover());
                                 icons::paint(ui.painter(), ir, c.icon, t.icon, t.icon_fill, t.accent);
-                                let b = ui.add_enabled(info.enabled, egui::Button::new(c.label).frame(false).shortcut_text(c.shortcut.unwrap_or("")));
+                                let b = ui.add_enabled(
+                                    info.enabled,
+                                    egui::Button::new(app.tr(c.label)).frame(false).shortcut_text(c.shortcut.unwrap_or("")),
+                                );
                                 crate::scenario::publish_handle(&format!("toolbar:{}", c.id), b.rect.center());
                                 if b.secondary_clicked() {
                                     menu = Some((b.interact_pointer_pos().unwrap_or(b.rect.left_bottom()), c.id));