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

app/lib/l10n/locales.dart

@@ -0,0 +1,89 @@
+/// The interface languages the client ships. Pure data: the settings page
+/// offers them, the loader reads their files, and the app tells Flutter which
+/// one is in use.
+library;
+
+/// One shipped language: the tag used in settings and in the asset path, and
+/// the id of its name in the locale files. A language is always named in its
+/// own words, so a reader who cannot read the current interface can still find
+/// theirs.
+typedef AppLocale = ({String tag, String labelId});
+
+/// Offered in this order, English first and then alphabetically by endonym.
+const appLocales = <AppLocale>[
+  (tag: 'en-US', labelId: 'language-en'),
+  (tag: 'de', labelId: 'language-de'),
+  (tag: 'es', labelId: 'language-es'),
+  (tag: 'fr', labelId: 'language-fr'),
+  (tag: 'he', labelId: 'language-he'),
+  (tag: 'ja', labelId: 'language-ja'),
+  (tag: 'pt', labelId: 'language-pt'),
+  (tag: 'ru', labelId: 'language-ru'),
+];
+
+/// What the reader's language setting holds when they have not chosen one.
+const systemLanguage = 'system';
+
+const fallbackLocale = 'en-US';
+
+/// Written right to left, so the whole interface mirrors.
+const rtlLocales = {'he'};
+
+bool isRightToLeft(String tag) => rtlLocales.contains(_languageOf(tag));
+
+/// The shipped locale for [tag], or null when nothing matches. A tag matches
+/// on its language alone, so a device set to de-AT reads the German files.
+String? matchLocale(String tag) {
+  final language = _languageOf(tag);
+  for (final locale in appLocales) {
+    if (locale.tag == tag) return locale.tag;
+  }
+  for (final locale in appLocales) {
+    if (_languageOf(locale.tag) == language) return locale.tag;
+  }
+  return null;
+}
+
+/// The first of [tags] the client ships, for resolving the device's list of
+/// preferred languages.
+String resolveLocale(Iterable<String> tags) {
+  for (final tag in tags) {
+    final match = matchLocale(tag);
+    if (match != null) return match;
+  }
+  return fallbackLocale;
+}
+
+String _languageOf(String tag) =>
+    tag.split(RegExp('[-_]')).first.toLowerCase();
+
+/// Which plural message a count needs, as a message id suffix. Zero is handled
+/// before this, so every language can say "none" in its own way.
+///
+/// Only the languages the client ships are described (CLDR cardinal rules):
+/// Russian separates two to four from the rest, Hebrew has a form for two,
+/// French and Portuguese treat zero and one alike, and Japanese has one form.
+String pluralSuffix(String tag, int count) {
+  final n = count.abs();
+  switch (_languageOf(tag)) {
+    case 'ru':
+      final mod10 = n % 10;
+      final mod100 = n % 100;
+      if (mod10 == 1 && mod100 != 11) return 'one';
+      if (mod10 >= 2 && mod10 <= 4 && !(mod100 >= 12 && mod100 <= 14)) {
+        return 'few';
+      }
+      return 'many';
+    case 'he':
+      if (n == 1) return 'one';
+      if (n == 2) return 'two';
+      return 'many';
+    case 'fr':
+    case 'pt':
+      return n <= 1 ? 'one' : 'many';
+    case 'ja':
+      return 'many';
+    default:
+      return n == 1 ? 'one' : 'many';
+  }
+}