feat(i18n): UI language selector (Settings → Appearance → Language) (T-462)

Make the localization usable: an app.locale select (English / Nederlands) in
the Appearance category, applied live by root_shell — it parses app.locale and
calls i18n.setLocale on boot + on settings change (setLocale is a no-op when
unchanged). nl_NL registered in availableLocales. The ui-design skill now
documents the locale-dir config and the rule to design for string-length
variation (translations run ~20% longer; never hard-size to the English label).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/theme_picker/src/extension.dart

@@ -4,7 +4,7 @@ import 'package:clide/builtin/theme_picker/src/settings_view.dart';
 import 'package:clide/builtin/theme_picker/src/theme_status_item.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
-import 'package:clide/widgets/widgets.dart' show kMonoFontSettingKey, kUiFontSettingKey;
+import 'package:clide/widgets/widgets.dart' show kLocaleSettingKey, kMonoFontSettingKey, kUiFontSettingKey;
 
 class ThemePickerExtension extends ClideExtension {
   @override
@@ -101,6 +101,26 @@ class ThemePickerExtension extends ClideExtension {
               ),
             ],
           ),
+          SettingsSection(
+            label: 'Language',
+            labelKey: 'settings.appearance.section.language',
+            fields: [
+              SettingsField(
+                key: kLocaleSettingKey,
+                kind: SettingsFieldKind.select,
+                label: 'Language',
+                labelKey: 'settings.appearance.field.language.label',
+                help: 'Language for the app interface; applies live.',
+                helpKey: 'settings.appearance.field.language.help',
+                defaultValue: 'en_US',
+                // Language names stay in their own language (no labelKey).
+                options: [
+                  SettingsOption(value: 'en_US', label: 'English'),
+                  SettingsOption(value: 'nl_NL', label: 'Nederlands'),
+                ],
+              ),
+            ],
+          ),
         ],
       ),
     ),

lib/main.dart

@@ -394,6 +394,10 @@ Future<void> main() async {
     bundledThemes: themes,
     i18nLoader: AssetCatalogLoader(bundle: rootBundle),
     preloadNamespaces: _tier0Namespaces,
+    // Languages the UI can switch to (Settings → Appearance, T-462). Each needs
+    // an assets/i18n/<locale>/ catalog folder; root_shell applies the persisted
+    // app.locale on boot.
+    availableLocales: const [Locale('en', 'US'), Locale('nl', 'NL')],
     autoStartDaemonClient: false,
     toolchain: toolchain,
     minLogLevel: bootLogLevel,

lib/src/shell/root_shell.dart

@@ -48,8 +48,9 @@ class RootShellState extends State<RootShell> {
     super.initState();
     _keyFocus = FocusNode()..requestFocus();
     widget.services.textZoom.addListener(_onZoom);
-    // Re-apply the UI font when its setting changes (T-460).
+    // Re-apply the UI font + language when their settings change (T-460/T-462).
     widget.services.settings.addListener(_onZoom);
+    _applyLocale(); // apply the persisted UI language at boot
     _globalSeq = SequenceMatcher(
       keymap: () => widget.services.keymap.keymap ?? Keymap(const []),
       context: () => widget.services.keymap.scope,
@@ -69,7 +70,20 @@ class RootShellState extends State<RootShell> {
     super.dispose();
   }
 
-  void _onZoom() => setState(() {});
+  void _onZoom() {
+    _applyLocale();
+    setState(() {});
+  }
+
+  /// Apply the persisted UI language (app.locale) to the i18n service. setLocale
+  /// is a no-op when the locale is unchanged, so this is safe on every tick.
+  void _applyLocale() {
+    final raw = widget.services.settings.get<String>(kLocaleSettingKey);
+    if (raw == null || raw.isEmpty) return;
+    final parts = raw.split(RegExp('[_-]'));
+    final loc = parts.length >= 2 ? Locale(parts[0], parts[1]) : Locale(parts[0]);
+    widget.services.i18n.setLocale(loc);
+  }
 
   @override
   Widget build(BuildContext context) {

lib/widgets/src/typography.dart

@@ -44,6 +44,10 @@ const List<String> clideUiFamilyFallback = [
 const String kUiFontSettingKey = 'app.ui.font';
 const String kMonoFontSettingKey = 'app.mono.font';
 
+/// Settings key for the UI language (Settings → Appearance, T-462). Value is a
+/// `lang_country` string (e.g. `en_US`, `nl_NL`); unset → the default locale.
+const String kLocaleSettingKey = 'app.locale';
+
 // ---------------------------------------------------------------------------
 // Monospace face — JetBrains Mono
 // ---------------------------------------------------------------------------