feat: ship the interface in eight languages, with per-language plurals and mirroring

app/lib/application/controllers/reader_settings_controller.dart

@@ -1,6 +1,7 @@
 import 'package:signals/signals.dart';
 
 import '../../database/database.dart';
+import '../../l10n/locales.dart';
 import '../fonts.dart';
 
 /// The reader's presentation choices (spec 8.2) plus the per-app limits they
@@ -25,6 +26,10 @@ class ReaderSettingsController {
   final themeMode = signal('system');
   final variant = signal('site');
 
+  /// The interface language: a shipped locale tag, or [systemLanguage] to take
+  /// whatever the device asks for.
+  final language = signal(systemLanguage);
+
   Future<void> load() async {
     final settings = await _db.readSettings();
     if (settings == null) return;
@@ -32,6 +37,7 @@ class ReaderSettingsController {
     fontScale.value = settings.fontScale;
     themeMode.value = settings.themeMode;
     variant.value = settings.variant;
+    language.value = settings.language;
   }
 
   Future<bool> setFont(String family) async {
@@ -71,10 +77,19 @@ class ReaderSettingsController {
     return true;
   }
 
+  Future<bool> setLanguage(String tag) async {
+    if (tag != systemLanguage && matchLocale(tag) == null) return false;
+    if (tag == language.value) return false;
+    language.value = tag;
+    await _persist();
+    return true;
+  }
+
   Future<void> _persist() => _db.saveSettings(
         font: font.value,
         fontScale: fontScale.value,
         themeMode: themeMode.value,
         variant: variant.value,
+        language: language.value,
       );
 }