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,
       );
 }

app/lib/application/internal_actions.dart

@@ -15,15 +15,18 @@ import 'pages/mews_urls.dart';
 class InternalActions {
   InternalActions({
     required MewsDatabase db,
+    required Future<void> Function() applyLanguage,
     required ReaderSettingsController settings,
     required BookmarksController bookmarks,
     required SubscriptionsController subscriptions,
   })  : _db = db,
+        _applyLanguage = applyLanguage,
         _settings = settings,
         _bookmarks = bookmarks,
         _subscriptions = subscriptions;
 
   final MewsDatabase _db;
+  final Future<void> Function() _applyLanguage;
   final ReaderSettingsController _settings;
   final BookmarksController _bookmarks;
   final SubscriptionsController _subscriptions;
@@ -37,6 +40,7 @@ class InternalActions {
       'set-size' => _setSize(args['step']),
       'set-theme' => _settings.setThemeMode(args['value'] ?? ''),
       'set-colors' => _settings.setVariant(args['value'] ?? ''),
+      'set-language' => _setLanguage(args['value']),
       'mark-read' => _markRead(args, read: true),
       'mark-unread' => _markRead(args, read: false),
       'refresh-subscriptions' => _refreshAll(),
@@ -52,6 +56,15 @@ class InternalActions {
     };
   }
 
+  /// Changing the language has to load its messages before the page is built
+  /// again, or the rerender would still read the old ones.
+  Future<bool> _setLanguage(String? tag) async {
+    if (tag == null || tag.isEmpty) return false;
+    if (!await _settings.setLanguage(tag)) return false;
+    await _applyLanguage();
+    return true;
+  }
+
   Future<bool> _setSize(String? step) => switch (step) {
     'up' => _settings.increaseSize(),
     'down' => _settings.decreaseSize(),

app/lib/application/pages/bookmarks_page.dart

@@ -20,6 +20,7 @@ Future<MewsPage> buildBookmarksPage(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       textForm(

app/lib/application/pages/history_page.dart

@@ -17,6 +17,7 @@ Future<MewsPage> buildHistoryPage(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       textForm(

app/lib/application/pages/hub_page.dart

@@ -23,6 +23,7 @@ Future<MewsPage> buildHubPage(
   return internalPage(
     url: url,
     title: strings.t('hub-title'),
+    strings: strings,
     body: [
       HeadingBlock(1, [MewsText(strings.t('hub-title'))]),
       itemList([

app/lib/application/pages/page_blocks.dart

@@ -196,25 +196,35 @@ ListBlock itemList(List<MewsListItem> items) =>
 /// Wraps blocks as an internal document. The marker makes the renderer treat
 /// it as a Mews page rather than prepending the "not a Mews page" notice, and
 /// no entries are offered so an internal page is never subscribable.
+///
+/// Unlike a fetched page, which declares its own language, one of ours is
+/// written in whichever language the reader set, so it takes its direction
+/// from that and mirrors when the interface does.
 MewsPage internalPage({
   required Uri url,
   required String title,
   required List<MewsBlock> body,
+  required Strings strings,
 }) {
   return MewsPage(
     url: url.toString(),
     specVersion: 'internal',
     title: title,
     body: body,
-    lang: 'en',
-    direction: TextDirection.ltr,
+    lang: strings.tag,
+    direction: strings.isRtl ? TextDirection.rtl : TextDirection.ltr,
     variant: MewsVariant.plain,
     entries: const [],
   );
 }
 
-/// Picks the message for [count]. Plurals are separate messages, so the
-/// choice happens here and Fluent formats the number.
+/// Picks the message for [count]. Plurals are separate messages rather than a
+/// rule inside one, so the choice happens here.
+///
+/// A language needing a form English does not have, such as Russian's "few"
+/// or Hebrew's "two", names it with the same id and that suffix. Those are
+/// derived from [many] rather than passed at every call, and a language
+/// without them falls through to [many].
 String countText(
   Strings strings,
   int count, {
@@ -222,11 +232,15 @@ String countText(
   required String one,
   required String many,
 }) {
-  return switch (count) {
-    0 => strings.t(none),
-    1 => strings.t(one),
-    _ => strings.t(many, {'count': count}),
-  };
+  if (count == 0) return strings.t(none);
+  final suffix = strings.pluralSuffixFor(count);
+  if (suffix == 'one') return strings.t(one, {'count': count});
+  if (suffix != 'many') {
+    final stem = many.substring(0, many.lastIndexOf('-'));
+    final text = strings.lookup('$stem-$suffix', {'count': count});
+    if (text != null) return text;
+  }
+  return strings.t(many, {'count': count});
 }
 
 /// YYYY-MM-DD, the shape a Mews entry link uses (spec 6.1).

app/lib/application/pages/posts_page.dart

@@ -23,6 +23,7 @@ Future<MewsPage> buildPostsPage(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       ParagraphBlock([

app/lib/application/pages/settings_page.dart

@@ -1,4 +1,5 @@
 import '../../l10n/strings.dart';
+import '../../l10n/locales.dart';
 import '../../mews/document.dart';
 import '../fonts.dart';
 import '../controllers/reader_settings_controller.dart'
@@ -33,6 +34,7 @@ Future<MewsPage> buildSettingsPage(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       HeadingBlock(2, [MewsText(strings.t('settings-font-heading'))]),
@@ -80,6 +82,22 @@ Future<MewsPage> buildSettingsPage(
             current: mode == settings.themeMode.value,
           ),
       ]),
+      // The language a reader wants may not be the one they can currently
+      // read, so each is named in its own words.
+      HeadingBlock(2, [MewsText(strings.t('settings-language-heading'))]),
+      choiceRow([
+        (
+          href: MewsUrls.action('set-language', {'value': systemLanguage}),
+          label: strings.t('settings-language-system'),
+          current: settings.language.value == systemLanguage,
+        ),
+        for (final locale in appLocales)
+          (
+            href: MewsUrls.action('set-language', {'value': locale.tag}),
+            label: strings.t(locale.labelId),
+            current: locale.tag == settings.language.value,
+          ),
+      ]),
       HeadingBlock(2, [MewsText(strings.t('settings-variant-heading'))]),
       choiceRow([
         for (final variant in ReaderSettingsController.variants)

app/lib/application/pages/subscriptions_page.dart

@@ -21,6 +21,7 @@ Future<MewsPage> buildSubscriptionsPage(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       textForm(

app/lib/application/pages/tags_page.dart

@@ -42,6 +42,7 @@ Future<MewsPage> _index(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       ...chrome(title: title, current: url, strings: strings),
       if (counts.isEmpty)
@@ -111,6 +112,7 @@ Future<MewsPage> _assign(
   return internalPage(
     url: url,
     title: title,
+    strings: strings,
     body: [
       HeadingBlock(1, [MewsText(title)]),
       ParagraphBlock([