diff --git a/apps/mobile/lib/app/app.dart b/apps/mobile/lib/app/app.dart
index 9c93910..5ebb60b 100644
--- a/apps/mobile/lib/app/app.dart
+++ b/apps/mobile/lib/app/app.dart
@@ -43,9 +43,30 @@ class _PhotoLiveAppState extends ConsumerState<PhotoLiveApp> {
     };
   }
 
+  /// 语言解析（P3 Task 10）：
+  /// - 显式选择（locale 非 null）优先，按 languageCode 匹配；
+  /// - system 档位下用系统 locale 匹配（zh_* → zh、en_* → en）；
+  /// - 均不匹配（如 fr）回退模板语言 zh——不依赖生成代码 supportedLocales
+  ///   的排序（gen-l10n 按字母序 en 在前，默认解析会回退 en，与模板不符）。
+  static Locale _resolveLocale(
+    Locale? deviceLocale,
+    Iterable<Locale> supportedLocales,
+  ) {
+    Locale? requested;
+    for (final supported in supportedLocales) {
+      if (supported.languageCode == deviceLocale?.languageCode) {
+        requested = supported;
+        break;
+      }
+    }
+    // 语言码不匹配 → 模板语言（l10n.yaml template-arb-file=app_zh.arb）。
+    return requested ?? const Locale('zh');
+  }
+
   @override
   Widget build(BuildContext context) {
-    final locale = ref.watch(localeSettingsProvider);
+    // select 只取 Locale?：未引起 locale 变化的状态更新不重建 MaterialApp。
+    final locale = ref.watch(localeSettingsProvider.select((c) => c.locale));
     // 引擎生命周期绑定（常驻 watch；账号在场 start / 离场 stop）。
     ref.watch(uploadEngineScopeProvider);
     return MaterialApp(
@@ -54,6 +75,7 @@ class _PhotoLiveAppState extends ConsumerState<PhotoLiveApp> {
       theme: AppTheme.light,
       darkTheme: AppTheme.dark,
       locale: locale,
+      localeResolutionCallback: _resolveLocale,
       supportedLocales: AppLocalizations.supportedLocales,
       localizationsDelegates: AppLocalizations.localizationsDelegates,
       routes: {
diff --git a/apps/mobile/lib/app/locale_controller.dart b/apps/mobile/lib/app/locale_controller.dart
index 784009d..7669263 100644
--- a/apps/mobile/lib/app/locale_controller.dart
+++ b/apps/mobile/lib/app/locale_controller.dart
@@ -1,16 +1,112 @@
-import 'package:flutter/material.dart';
-import 'package:flutter_riverpod/flutter_riverpod.dart';
-
-/// App 语言设置（脚手架）。
+/// App 语言设置（P3 计划 Task 10 完整版）。
 ///
-/// `null` = 跟随系统；Task 10 补齐：持久化（settings 存储）、
-/// 设置页交互与全部页面的 l10n 覆盖检查。
-class LocaleSettings extends Notifier<Locale?> {
-  @override
-  Locale? build() => null;
+/// 三档选择（跟随系统 / 中文 / English），持久化于 shared_preferences
+/// 键 `app.locale`（'system'|'zh'|'en'）。main() 在 runApp 前读持久值
+/// 注入 Notifier 初始状态——首帧即正确语言（无 system→选择闪变）；
+/// 持久值非法（旧版本/手改损坏）回退 system。
+///
+/// 存储读写失败均不阻断：读失败按 system；写失败保留本次会话状态
+/// （仅丢失记忆，下次启动回退旧值）。
+library;
 
-  void setLocale(Locale? locale) => state = locale;
+import 'package:flutter/widgets.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// 语言档位（枚举值顺序即设置页选项顺序）。
+enum AppLocaleChoice {
+  /// 跟随系统（MaterialApp.locale=null，交由 localeResolutionCallback 解析）。
+  system('system'),
+
+  /// 固定中文。
+  zh('zh'),
+
+  /// 固定英文。
+  en('en');
+
+  const AppLocaleChoice(this.persistedValue);
+
+  /// 持久化字符串（shared_preferences `app.locale` 的值域）。
+  final String persistedValue;
+
+  /// 对应 [MaterialApp.locale]；system 为 null。
+  Locale? get locale => switch (this) {
+        AppLocaleChoice.system => null,
+        AppLocaleChoice.zh => const Locale('zh'),
+        AppLocaleChoice.en => const Locale('en'),
+      };
+
+  /// 持久值解析；null/未知值回退 [system]（前向兼容未来新增档位）。
+  static AppLocaleChoice fromPersisted(String? value) =>
+      AppLocaleChoice.values.firstWhere(
+        (choice) => choice.persistedValue == value,
+        orElse: () => AppLocaleChoice.system,
+      );
+}
+
+/// 存储抽象（测试注入内存实现；生产=shared_preferences 单键）。
+abstract class LocalePrefsStore {
+  Future<String?> read();
+  Future<void> write(String value);
+}
+
+/// [LocalePrefsStore] 的 shared_preferences 生产实现。
+class SharedPrefsLocaleStore implements LocalePrefsStore {
+  const SharedPrefsLocaleStore();
+
+  static const String kKey = 'app.locale';
+
+  @override
+  Future<String?> read() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(kKey);
+  }
+
+  @override
+  Future<void> write(String value) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(kKey, value);
+  }
+}
+
+/// 语言设置 Notifier：状态为当前档位，[localeSettingsProvider] 全局唯一。
+///
+/// 生产经 main() 以持久值构造（overrideWith 注入，见 main.dart）；
+/// 测试可注入初始档位与内存存储。
+class LocaleSettings extends Notifier<AppLocaleChoice> {
+  LocaleSettings({
+    this.initial = AppLocaleChoice.system,
+    LocalePrefsStore? store,
+  }) : _store = store ?? const SharedPrefsLocaleStore();
+
+  /// 启动注入的初始档位（main() 以持久值构造）。
+  final AppLocaleChoice initial;
+  final LocalePrefsStore _store;
+
+  @override
+  AppLocaleChoice build() => initial;
+
+  /// 切换档位：状态立即生效（MaterialApp 即刻重建）；持久化尽力而为。
+  Future<void> setChoice(AppLocaleChoice choice) async {
+    state = choice;
+    try {
+      await _store.write(choice.persistedValue);
+    } on Object {
+      // 写失败不回滚：语言记忆属锦上添花，不值得打断交互。
+    }
+  }
+
+  /// 启动读取持久档位（main() runApp 前调用；失败按 system）。
+  static Future<AppLocaleChoice> readPersisted([
+    LocalePrefsStore store = const SharedPrefsLocaleStore(),
+  ]) async {
+    try {
+      return AppLocaleChoice.fromPersisted(await store.read());
+    } on Object {
+      return AppLocaleChoice.system;
+    }
+  }
 }
 
 final localeSettingsProvider =
-    NotifierProvider<LocaleSettings, Locale?>(LocaleSettings.new);
+    NotifierProvider<LocaleSettings, AppLocaleChoice>(LocaleSettings.new);
diff --git a/apps/mobile/lib/main.dart b/apps/mobile/lib/main.dart
index ef4933b..7b32389 100644
--- a/apps/mobile/lib/main.dart
+++ b/apps/mobile/lib/main.dart
@@ -2,7 +2,21 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'app/app.dart';
+import 'app/locale_controller.dart';
 
-void main() {
-  runApp(const ProviderScope(child: PhotoLiveApp()));
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  // 启动先读语言偏好再 runApp：首帧即正确语言（无「system 闪变」，
+  // P3 Task 10）。读取失败由 readPersisted 内部兜底为 system。
+  final localeChoice = await LocaleSettings.readPersisted();
+  runApp(
+    ProviderScope(
+      overrides: [
+        localeSettingsProvider.overrideWith(
+          () => LocaleSettings(initial: localeChoice),
+        ),
+      ],
+      child: const PhotoLiveApp(),
+    ),
+  );
 }
diff --git a/apps/mobile/lib/pages/home_page.dart b/apps/mobile/lib/pages/home_page.dart
index a05c40f..90825fa 100644
--- a/apps/mobile/lib/pages/home_page.dart
+++ b/apps/mobile/lib/pages/home_page.dart
@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../app/locale_controller.dart';
 import '../core/api_client.dart';
 import '../features/uploads/engine_scope.dart';
 import '../l10n/app_localizations.dart';
@@ -56,10 +57,53 @@ class _HomePageState extends State<HomePage> {
   }
 }
 
-/// 设置页（P3 Task 3：修改密码入口 + 退出登录；Task 10 补语言切换持久化）。
+/// 设置页（P3 Task 3：修改密码入口 + 退出登录；Task 10：语言切换）。
 class SettingsTab extends ConsumerWidget {
   const SettingsTab({super.key});
 
+  /// 档位 → 展示名（语言名固定以自身语言书写，不随当前界面语言变化）。
+  static String _choiceLabel(AppLocalizations l10n, AppLocaleChoice choice) =>
+      switch (choice) {
+        AppLocaleChoice.system => l10n.localeFollowSystem,
+        AppLocaleChoice.zh => l10n.localeChinese,
+        AppLocaleChoice.en => l10n.localeEnglish,
+      };
+
+  Future<void> _pickLocale(
+    BuildContext context,
+    WidgetRef ref,
+    AppLocaleChoice current,
+  ) {
+    return showModalBottomSheet<void>(
+      context: context,
+      showDragHandle: true,
+      builder: (sheetContext) {
+        final l10n = AppLocalizations.of(sheetContext);
+        // 打开时刻的快照即可：选中即 pop，无滞留期间的陈旧选中态。
+        return SafeArea(
+          child: RadioGroup<AppLocaleChoice>(
+            groupValue: current,
+            onChanged: (choice) {
+              if (choice == null) return;
+              ref.read(localeSettingsProvider.notifier).setChoice(choice);
+              Navigator.of(sheetContext).pop();
+            },
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                for (final choice in AppLocaleChoice.values)
+                  RadioListTile<AppLocaleChoice>(
+                    value: choice,
+                    title: Text(_choiceLabel(l10n, choice)),
+                  ),
+              ],
+            ),
+          ),
+        );
+      },
+    );
+  }
+
   Future<void> _logout(BuildContext context, WidgetRef ref) async {
     // api.logout 尽力而为（服务端撤销失败也照常本地清会话——客户端内已处理）。
     await ref.read(apiClientProvider).logout();
@@ -72,6 +116,7 @@ class SettingsTab extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final l10n = AppLocalizations.of(context);
+    final choice = ref.watch(localeSettingsProvider);
     return Scaffold(
       appBar: AppBar(title: Text(l10n.settings)),
       body: ListView(
@@ -79,7 +124,8 @@ class SettingsTab extends ConsumerWidget {
           ListTile(
             leading: const Icon(Icons.language),
             title: Text(l10n.language),
-            subtitle: Text(l10n.localeFollowSystem),
+            subtitle: Text(_choiceLabel(l10n, choice)),
+            onTap: () => _pickLocale(context, ref, choice),
           ),
           ListTile(
             leading: const Icon(Icons.password_outlined),
diff --git a/apps/mobile/test/app/locale_test.dart b/apps/mobile/test/app/locale_test.dart
new file mode 100644
index 0000000..8d233e3
--- /dev/null
+++ b/apps/mobile/test/app/locale_test.dart
@@ -0,0 +1,172 @@
+/// App 内语言切换测试（P3 计划 Task 10）。
+///
+/// 覆盖：
+/// - 启动：持久档位注入 Notifier → 首帧即对应语言（无闪变）；
+/// - 切换：setChoice → MaterialApp 即刻重建（登录页文案随切）+ 写持久层；
+/// - 解析矩阵（system 档）：系统 zh → zh、系统 en → en、系统其他 →
+///   回退模板语言 zh（l10n.yaml template=app_zh.arb）；
+/// - 显式选择优先于系统 locale；
+/// - 持久值解析容错（null/非法值/存储异常 → system）。
+library;
+
+import 'package:dio/dio.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:photo_live/app/app.dart';
+import 'package:photo_live/app/locale_controller.dart';
+import 'package:photo_live/core/api_client.dart';
+
+import '../helpers/fakes.dart';
+
+/// 写入即抛的存储替身（持久化失败容错断言用）。
+class ThrowingWriteLocaleStore extends FakeLocaleStore {
+  @override
+  Future<void> write(String value) async => throw StateError('disk full');
+}
+
+void main() {
+  group('持久值解析（纯逻辑）', () {
+    test('fromPersisted：合法值/空值/非法值', () {
+      expect(AppLocaleChoice.fromPersisted('system'), AppLocaleChoice.system);
+      expect(AppLocaleChoice.fromPersisted('zh'), AppLocaleChoice.zh);
+      expect(AppLocaleChoice.fromPersisted('en'), AppLocaleChoice.en);
+      expect(AppLocaleChoice.fromPersisted(null), AppLocaleChoice.system);
+      expect(AppLocaleChoice.fromPersisted('fr'), AppLocaleChoice.system,
+          reason: '未来档位/手改损坏的旧值一律回退 system');
+    });
+
+    test('readPersisted：读取异常 → system（启动不因存储故障卡死）', () async {
+      final store = ThrowingWriteLocaleStore();
+      final choice = await LocaleSettings.readPersisted(store);
+      expect(choice, AppLocaleChoice.system);
+    });
+
+    test('setChoice 写失败不回滚本次会话状态', () async {
+      final container = ProviderContainer(
+        overrides: [
+          localeSettingsProvider.overrideWith(
+            () => LocaleSettings(store: ThrowingWriteLocaleStore()),
+          ),
+        ],
+      );
+      addTearDown(container.dispose);
+      await container
+          .read(localeSettingsProvider.notifier)
+          .setChoice(AppLocaleChoice.en);
+      expect(container.read(localeSettingsProvider), AppLocaleChoice.en);
+    });
+  });
+
+  group('PhotoLiveApp 语言接线（widget）', () {
+    late FakeSessionStore session;
+    late ApiClient client;
+
+    setUp(() {
+      // 无会话：App 停在登录页（文案断言载体）；客户端拒绝一切请求
+      //（语言测试不触网，仅为满足 Provider 依赖图）。
+      session = FakeSessionStore();
+      client = ApiClient(
+        sessionStore: session,
+        dio: Dio(BaseOptions(baseUrl: 'http://test.local'))
+          ..httpClientAdapter =
+              FakeAdapter((o) => throw StateError('不应发起请求: ${o.path}')),
+      );
+    });
+
+    Future<ProviderContainer> pumpApp(
+      WidgetTester tester, {
+      AppLocaleChoice initial = AppLocaleChoice.system,
+      FakeLocaleStore? localeStore,
+    }) async {
+      final store = localeStore ?? FakeLocaleStore();
+      await tester.pumpWidget(
+        ProviderScope(
+          overrides: [
+            ...pageTestOverrides(session: session, client: client),
+            localeSettingsProvider.overrideWith(
+              () => LocaleSettings(initial: initial, store: store),
+            ),
+          ],
+          child: const PhotoLiveApp(),
+        ),
+      );
+      await tester.pumpAndSettle();
+      return ProviderScope.containerOf(
+        tester.element(find.byType(PhotoLiveApp)),
+      );
+    }
+
+    testWidgets('持久档位 zh → 首帧中文文案', (tester) async {
+      final store = FakeLocaleStore()..value = 'zh';
+      await pumpApp(tester,
+          initial: AppLocaleChoice.zh, localeStore: store);
+      expect(find.text('登录'), findsOneWidget);
+      expect(find.text('Sign in'), findsNothing);
+    });
+
+    testWidgets('切换 en → 登录页即刻英文 + 持久化 en', (tester) async {
+      final store = FakeLocaleStore();
+      final container = await pumpApp(tester,
+          initial: AppLocaleChoice.zh, localeStore: store);
+      expect(find.text('登录'), findsOneWidget);
+
+      await container
+          .read(localeSettingsProvider.notifier)
+          .setChoice(AppLocaleChoice.en);
+      await tester.pumpAndSettle();
+
+      expect(find.text('Sign in'), findsOneWidget,
+          reason: 'MaterialApp 已随 provider 重建');
+      expect(find.text('登录'), findsNothing);
+      expect(store.writes, ['en'], reason: '写入持久层');
+    });
+
+    testWidgets('切换回 system → 交还系统 locale', (tester) async {
+      tester.platformDispatcher.localesTestValue = const [Locale('en')];
+      addTearDown(tester.platformDispatcher.clearLocalesTestValue);
+      final container = await pumpApp(tester, initial: AppLocaleChoice.zh);
+      expect(find.text('登录'), findsOneWidget);
+
+      await container
+          .read(localeSettingsProvider.notifier)
+          .setChoice(AppLocaleChoice.system);
+      await tester.pumpAndSettle();
+
+      expect(find.text('Sign in'), findsOneWidget, reason: '系统 en 接管');
+    });
+
+    testWidgets('解析矩阵：system + 系统 zh → zh', (tester) async {
+      tester.platformDispatcher.localesTestValue = const [Locale('zh')];
+      addTearDown(tester.platformDispatcher.clearLocalesTestValue);
+      await pumpApp(tester);
+      expect(find.text('登录'), findsOneWidget);
+    });
+
+    testWidgets('解析矩阵：system + 系统 en → en', (tester) async {
+      tester.platformDispatcher.localesTestValue = const [Locale('en')];
+      addTearDown(tester.platformDispatcher.clearLocalesTestValue);
+      await pumpApp(tester);
+      expect(find.text('Sign in'), findsOneWidget);
+    });
+
+    testWidgets('解析矩阵：system + 系统其他（fr）→ 回退模板语言 zh',
+        (tester) async {
+      // Localizations 解析用 platformDispatcher.locales（首选语言列表），
+      // 单值 localeTestValue 不影响它——须整体注入列表。
+      tester.platformDispatcher.localesTestValue = const [Locale('fr')];
+      addTearDown(tester.platformDispatcher.clearLocalesTestValue);
+      await pumpApp(tester);
+      expect(find.text('登录'), findsOneWidget,
+          reason: '不依赖 supportedLocales 排序（默认解析会回退 en）');
+    });
+
+    testWidgets('显式选择优先于系统 locale（选 en + 系统 zh → en）',
+        (tester) async {
+      tester.platformDispatcher.localesTestValue = const [Locale('zh')];
+      addTearDown(tester.platformDispatcher.clearLocalesTestValue);
+      await pumpApp(tester, initial: AppLocaleChoice.en);
+      expect(find.text('Sign in'), findsOneWidget);
+    });
+  });
+}
diff --git a/apps/mobile/test/helpers/fakes.dart b/apps/mobile/test/helpers/fakes.dart
index 80ef64f..aee8342 100644
--- a/apps/mobile/test/helpers/fakes.dart
+++ b/apps/mobile/test/helpers/fakes.dart
@@ -10,6 +10,7 @@ import 'dart:convert';
 
 import 'package:dio/dio.dart';
 import 'package:drift/drift.dart' show driftRuntimeOptions;
+import 'package:photo_live/app/locale_controller.dart';
 import 'package:photo_live/core/api_client.dart';
 import 'package:photo_live/core/session_store.dart';
 import 'package:photo_live/features/uploads/providers.dart';
@@ -29,6 +30,24 @@ class FakeSessionStore implements SessionStore {
   Future<void> clear() async => session = null;
 }
 
+/// 内存语言偏好存储（测试替身，P3 Task 10）。
+class FakeLocaleStore implements LocalePrefsStore {
+  /// 当前持久值（read 返回）；null = 未设置（跟随系统）。
+  String? value;
+
+  /// 按序记录全部写入（断言持久化调用用）。
+  final List<String> writes = <String>[];
+
+  @override
+  Future<String?> read() async => value;
+
+  @override
+  Future<void> write(String value) async {
+    writes.add(value);
+    this.value = value;
+  }
+}
+
 /// Widget 测试通用 overrides（P3 Task 9 起，登录页/主壳会触达
 /// sessionAccountProvider 与任务库 Provider——真实 flutter_secure_storage
 /// 与 path_provider 的平台通道在测试环境挂起，凡 pump 到相关页面均需
diff --git a/apps/mobile/test/l10n/arb_completeness_test.dart b/apps/mobile/test/l10n/arb_completeness_test.dart
new file mode 100644
index 0000000..efe4cb5
--- /dev/null
+++ b/apps/mobile/test/l10n/arb_completeness_test.dart
@@ -0,0 +1,121 @@
+/// arb 完整性单测（P3 计划 Task 10：漏译检查）。
+///
+/// zh（模板）/en 两份 arb 直接按 JSON 解析比对：
+/// 1. 消息键集合相等（任一侧新增/删除键即失败——杜绝漏译）；
+/// 2. 每条消息的占位符 token 集合相等（en 用了 {count} 而 zh 没有即失败，
+///    反之亦然——杜绝运行时 Intl 占位符缺失/多参崩溃）；
+/// 3. `@key` 元数据键必须对应真实消息键，且其 placeholders 名称与
+///    消息体中提取的 token 一致（两侧一致）。
+library;
+
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+Map<String, Object?> _loadArb(String fileName) {
+  final file = File('lib/l10n/$fileName');
+  if (!file.existsSync()) {
+    fail('arb 文件缺失: ${file.path}');
+  }
+  return jsonDecode(file.readAsStringSync()) as Map<String, Object?>;
+}
+
+/// `{name}` 占位符 token 提取（ICU 复数/选择子句的内层同名 token 同样
+/// 命中；`{count, plural,` 等带参数形式不误报为 token）。
+Set<String> _placeholdersOf(String message) =>
+    RegExp(r'\{([a-zA-Z][a-zA-Z0-9_]*)\}')
+        .allMatches(message)
+        .map((m) => m.group(1)!)
+        .toSet();
+
+Set<String> _messageKeys(Map<String, Object?> arb) =>
+    arb.keys.where((k) => !k.startsWith('@')).toSet();
+
+void main() {
+  final zh = _loadArb('app_zh.arb');
+  final en = _loadArb('app_en.arb');
+  final zhKeys = _messageKeys(zh);
+  final enKeys = _messageKeys(en);
+
+  test('zh/en 消息键集合相等（漏译防护）', () {
+    final zhOnly = zhKeys.difference(enKeys).toList()..sort();
+    final enOnly = enKeys.difference(zhKeys).toList()..sort();
+    expect(zhOnly, isEmpty, reason: 'en 缺少这些键: $zhOnly');
+    expect(enOnly, isEmpty, reason: 'zh 缺少这些键: $enOnly');
+  });
+
+  test('@@locale 元数据正确', () {
+    expect(zh['@@locale'], 'zh');
+    expect(en['@@locale'], 'en');
+  });
+
+  test('每条消息的占位符 token 两侧一致', () {
+    final mismatched = <String>[];
+    for (final key in zhKeys.intersection(enKeys)) {
+      final zhValue = zh[key];
+      final enValue = en[key];
+      if (zhValue is! String || enValue is! String) continue;
+      final zhTokens = _placeholdersOf(zhValue);
+      final enTokens = _placeholdersOf(enValue);
+      final sameSet = zhTokens.length == enTokens.length &&
+          zhTokens.containsAll(enTokens);
+      if (!sameSet) {
+        mismatched.add(
+          '$key: zh${zhTokens.toList()} vs en${enTokens.toList()}',
+        );
+      }
+    }
+    expect(mismatched, isEmpty, reason: '占位符不一致:\n${mismatched.join('\n')}');
+  });
+
+  test('@key 元数据与消息键对应且 placeholders 名称一致', () {
+    for (final arb in [zh, en]) {
+      final locale = arb['@@locale'] as String?;
+      final messageKeys = _messageKeys(arb);
+      final metaKeys = arb.keys
+          .where((k) => k.startsWith('@') && k != '@@locale')
+          .toSet();
+      final dangling =
+          {for (final k in metaKeys) k.substring(1)}
+              .difference(messageKeys)
+              .toList()
+            ..sort();
+      expect(
+        dangling,
+        isEmpty,
+        reason: '$locale 存在无对应消息键的元数据: $dangling',
+      );
+
+      for (final metaKey in metaKeys) {
+        final key = metaKey.substring(1);
+        final meta = arb[metaKey];
+        if (meta is! Map) continue;
+        final declared = (meta['placeholders'] as Map<Object?, Object?>?)
+            ?.keys
+            .whereType<String>()
+            .toSet();
+        if (declared == null) continue;
+        final message = arb[key];
+        if (message is! String) continue;
+        expect(
+          _placeholdersOf(message),
+          declared,
+          reason: '$locale `$key` 消息体 token 与 @placeholders 声明不一致',
+        );
+      }
+    }
+  });
+
+  test('生成的 AppLocalizations 覆盖全部 arb 消息键', () {
+    // 拦截「改了 arb 忘了重新生成本地化代码」的漂移（flutter test 本身
+    // 会触发生成，此处守护绕过 flutter 工具链的手工改动）。
+    final generated =
+        File('lib/l10n/app_localizations.dart').readAsStringSync();
+    final missing = zhKeys
+        .where((key) => !RegExp('\\b$key\\b').hasMatch(generated))
+        .toList()
+      ..sort();
+    expect(missing, isEmpty, reason: '生成代码缺少这些键: $missing');
+  });
+}
diff --git a/apps/mobile/test/pages/settings_tab_test.dart b/apps/mobile/test/pages/settings_tab_test.dart
index 56b22b2..fd958fc 100644
--- a/apps/mobile/test/pages/settings_tab_test.dart
+++ b/apps/mobile/test/pages/settings_tab_test.dart
@@ -8,6 +8,7 @@ import 'package:dio/dio.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:photo_live/app/locale_controller.dart';
 import 'package:photo_live/core/api_client.dart';
 import 'package:photo_live/core/session_store.dart';
 import 'package:photo_live/l10n/app_localizations.dart';
@@ -35,13 +36,18 @@ void main() {
           ..httpClientAdapter = adapter,
       );
 
-  Future<void> pumpHome(WidgetTester tester, ApiClient client) async {
+  Future<void> pumpHome(
+    WidgetTester tester,
+    ApiClient client, {
+    List<dynamic> extraOverrides = const <dynamic>[],
+  }) async {
     await tester.pumpWidget(
       ProviderScope(
         // Task 9 起主壳含队列页（触达 sessionAccountProvider/TaskStore），
         // 真实平台通道在测试环境挂起，必须注入替身。
         overrides: [
           ...pageTestOverrides(session: store, client: client),
+          ...extraOverrides,
         ],
         child: MaterialApp(
           localizationsDelegates: AppLocalizations.localizationsDelegates,
@@ -92,4 +98,39 @@ void main() {
     expect(find.byType(LoginPage), findsOneWidget);
     expect(find.byType(HomePage), findsNothing, reason: '主壳已出栈');
   });
+
+  testWidgets('语言切换：底部弹层三选一 → 更新档位 + 持久化（P3 Task 10）',
+      (tester) async {
+    final adapter = FakeAdapter((_) => throw StateError('不应发起请求'));
+    final localeStore = FakeLocaleStore();
+    await pumpHome(
+      tester,
+      clientOf(adapter),
+      extraOverrides: [
+        localeSettingsProvider.overrideWith(
+          () => LocaleSettings(store: localeStore),
+        ),
+      ],
+    );
+
+    // 本 harness 的 MaterialApp 固定 en（live 全 App 重建在
+    // app/locale_test.dart 的 PhotoLiveApp 用例覆盖）。
+    expect(find.text('Language'), findsOneWidget);
+    expect(find.text('Follow system'), findsOneWidget, reason: '默认档位副标题');
+
+    await tester.tap(find.text('Language'));
+    await tester.pumpAndSettle();
+
+    // 弹层三选项（语言名以自身语言书写，跨 locale 不变）。
+    // 「Follow system」出现两次：主档 tile 副标题 + 弹层选项。
+    expect(find.text('Follow system'), findsNWidgets(2));
+    expect(find.text('中文'), findsOneWidget);
+    expect(find.text('English'), findsOneWidget);
+
+    await tester.tap(find.text('中文'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('中文'), findsOneWidget, reason: '副标题更新为当前档位');
+    expect(localeStore.writes, ['zh'], reason: '选择写入 app.locale');
+  });
 }