db: Store language global setting in database

An alternative to implementing _fromLanguageTag is using the
LocaleParser from package:intl/locale.dart.  In this case though,
the values to parse always come from `toLanguageTag`, so we can
utilize that knowledge to get away with a more simple implementation,
that should also be handy upstream.

The locale stored in the database is not guaranteed to be one of our
supported value, especially if we need to rename locales in the future.
See discussion:
  #1513 (comment)

Author: PIG208

lib/model/database.dart

@@ -1,3 +1,5 @@
+import 'dart:ui';
+
 import 'package:drift/drift.dart';
 import 'package:drift/internal/versioned_schema.dart';
 import 'package:drift/remote.dart';
@@ -30,6 +32,9 @@ class GlobalSettings extends Table {
   Column<String> get markReadOnScroll => textEnum<MarkReadOnScrollSetting>()
     .nullable()();
 
+  Column<String> get language => text().map(const LocaleConverter())
+    .nullable()();
+
   // If adding a new column to this table, consider whether [BoolGlobalSettings]
   // can do the job instead (by adding a value to the [BoolGlobalSetting] enum).
   // That way is more convenient, when it works, because
@@ -125,7 +130,7 @@ class AppDatabase extends _$AppDatabase {
   //    information on using the build_runner.
   //  * Write a migration in `_migrationSteps` below.
   //  * Write tests.
-  static const int latestSchemaVersion = 8; // See note.
+  static const int latestSchemaVersion = 9; // See note.
 
   @override
   int get schemaVersion => latestSchemaVersion;
@@ -188,6 +193,9 @@ class AppDatabase extends _$AppDatabase {
       await m.addColumn(schema.globalSettings,
         schema.globalSettings.markReadOnScroll);
     },
+    from8To9: (m, schema) async {
+      await m.addColumn(schema.globalSettings, schema.globalSettings.language);
+    }
   );
 
   Future<void> _createLatestSchema(Migrator m) async {
@@ -257,3 +265,55 @@ class AppDatabase extends _$AppDatabase {
 }
 
 class AccountAlreadyExistsException implements Exception {}
+
+class LocaleConverter extends TypeConverter<Locale, String> {
+  const LocaleConverter();
+
+  /// Parse a Unicode BCP 47 Language Identifier into [Locale].
+  ///
+  /// Throw when it fails to convert [languageTag] into a [Locale].
+  ///
+  /// This supports parsing a Unicode Language Identifier returned from
+  /// [Locale.toLanguageTag].
+  ///
+  /// This implementation refers to a part of
+  /// [this EBNF grammar](https://www.unicode.org/reports/tr35/#Unicode_language_identifier),
+  /// assuming the identifier is valid without
+  /// [unicode_variant_subtag](https://www.unicode.org/reports/tr35/#unicode_variant_subtag).
+  ///
+  /// This doesn't check if the [languageTag] is a valid identifier, (i.e., when
+  /// this returns without errors, the identifier is not necessarily
+  /// syntactically well-formed or valid).
+  // TODO(upstream): send this as a factory Locale.fromLanguageTag
+  //   https://github.com/flutter/flutter/issues/143491
+  Locale _fromLanguageTag(String languageTag) {
+    final subtags = languageTag.replaceAll('_', '-').split('-');
+
+    return switch (subtags) {
+      [final language, final script, final region] =>
+        Locale.fromSubtags(
+          languageCode: language, scriptCode: script, countryCode: region),
+
+      [final language, final script] when script.length == 4 =>
+        Locale.fromSubtags(languageCode: language, scriptCode: script),
+
+      [final language, final region] =>
+        Locale(language, region),
+
+      [final language] =>
+        Locale(language),
+
+      _ => throw ArgumentError.value(languageTag, 'languageTag'),
+    };
+  }
+
+  @override
+  Locale fromSql(String fromDb) {
+    return _fromLanguageTag(fromDb);
+  }
+
+  @override
+  String toSql(Locale value) {
+    return value.toLanguageTag();
+  }
+}