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

app/lib/presentation/app.dart

@@ -1,7 +1,10 @@
 import 'dart:async' show unawaited;
 
+import 'dart:ui' show PlatformDispatcher;
+
 import 'package:actors/actors.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:signals/signals_flutter.dart' show SignalBuilder;
 
 import '../application/actors/fetch_actor.dart';
@@ -13,6 +16,7 @@ import '../application/fonts.dart';
 import '../application/internal_actions.dart';
 import '../application/internal_pages.dart';
 import '../database/database.dart';
+import '../l10n/locales.dart';
 import '../l10n/strings.dart';
 import '../mews/document.dart';
 import 'reader/palettes.dart';
@@ -38,6 +42,7 @@ class AppDependencies {
         strings: this.strings);
     internalActions = InternalActions(
       db: db,
+      applyLanguage: applyLanguage,
       settings: settings,
       bookmarks: bookmarks,
       subscriptions: subscriptions,
@@ -65,8 +70,20 @@ class AppDependencies {
   late final InternalPages internalPages;
   late final InternalActions internalActions;
 
+  /// The language to read the interface in: the reader's choice, or the best
+  /// match for what the device asks for.
+  String get activeLocale => settings.language.value == systemLanguage
+      ? resolveLocale(
+          PlatformDispatcher.instance.locales.map((l) => l.toLanguageTag()))
+      : settings.language.value;
+
+  /// Loads the chosen language's messages. Called at startup and whenever the
+  /// reader changes the setting.
+  Future<void> applyLanguage() => strings.use(activeLocale);
+
   Future<void> start() async {
     await settings.load();
+    await applyLanguage();
     await bookmarks.load();
     await subscriptions.load();
     await browser.restoreLastSession();
@@ -129,13 +146,30 @@ class _ThemedApp extends StatelessWidget {
       };
       final settings = deps.settings;
       final font = settings.font.value;
+      // Read so the whole app rebuilds when the reader changes language.
+      settings.language.value;
       final page = deps.browser.currentPage.value;
       final variant = settings.variant.value == 'site'
           ? (page?.variant ?? MewsVariant.plain)
           : pageVariantFromName(settings.variant.value);
 
+      // Flutter's own widgets and the text direction follow the same
+      // language, so a right-to-left interface mirrors without us asking.
+      final locale = deps.activeLocale;
       return MaterialApp(
         title: 'Mews',
+        locale: Locale(locale.split('-').first,
+            locale.contains('-') ? locale.split('-').last : null),
+        supportedLocales: [
+          for (final l in appLocales)
+            Locale(l.tag.split('-').first,
+                l.tag.contains('-') ? l.tag.split('-').last : null),
+        ],
+        localizationsDelegates: const [
+          GlobalMaterialLocalizations.delegate,
+          GlobalWidgetsLocalizations.delegate,
+          GlobalCupertinoLocalizations.delegate,
+        ],
         themeMode: mode,
         theme: _themeFor(
             paletteFor(pageVariant: variant, readerVariant: 'site', dark: false),