Localizing Flutter Applications

Contents

• Core Configuration
• Defining ARB Resources
• App Integration
• Advanced Formatting
• Workflows
• Troubleshooting & Gotchas

Core Configuration

Configure the project to support code generation for localizations.

1. Add required dependencies to pubspec.yaml:

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: any

flutter:
  generate: true # Required for l10n code generation

2. Create an l10n.yaml file in the project root to configure the gen-l10n tool:

arb-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
# Optional: Set to false to generate files into lib/ instead of the synthetic package
# synthetic-package: false 

Defining ARB Resources

Store localized strings in Application Resource Bundle (.arb) files within the configured arb-dir.

Create the template file (e.g., lib/l10n/app_en.arb):

{
  "helloWorld": "Hello World!",
  "@helloWorld": {
    "description": "The conventional newborn programmer greeting"
  }
}

Create translation files (e.g., lib/l10n/app_es.arb):

{
  "helloWorld": "¡Hola Mundo!"
}

App Integration

Initialize the Localizations widget by configuring the root MaterialApp or CupertinoApp.

import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart'; // Adjust import if synthetic-package is false

return MaterialApp(
  title: 'Localized App',
  localizationsDelegates: const [
    AppLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
    GlobalCupertinoLocalizations.delegate,
  ],
  supportedLocales: const [
    Locale('en'), // English
    Locale('es'), // Spanish
  ],
  home: const MyHomePage(),
);

Access localized values in the widget tree using the generated AppLocalizations class:

Text(AppLocalizations.of(context)!.helloWorld)

Note: If using WidgetsApp instead of MaterialApp, omit GlobalMaterialLocalizations.delegate.

Advanced Formatting

Use placeholders, plurals, and selects for dynamic content. Define parameters in the @key metadata.

Placeholders

"hello": "Hello {userName}",
"@hello": {
  "description": "A message with a single parameter",
  "placeholders": {
    "userName": {
      "type": "String",
      "example": "Bob"
    }
  }
}

Plurals

"nWombats": "{count, plural, =0{no wombats} =1{1 wombat} other{{count} wombats}}",
"@nWombats": {
  "placeholders": {
    "count": {
      "type": "num",
      "format": "compact"
    }
  }
}

Selects (Gender/Enums)

"pronoun": "{gender, select, male{he} female{she} other{they}}",
"@pronoun": {
  "placeholders": {
    "gender": {
      "type": "String"
    }
  }
}

Dates and Numbers

Use format and optionalParameters to leverage intl formatting.

"dateMessage": "Date: {date}",
"@dateMessage": {
  "placeholders": {
    "date": {
      "type": "DateTime",
      "format": "yMd"
    }
  }
}

Workflows

Task Progress: Adding a New Language

Copy this checklist to track progress when introducing a new locale.

• Create a new .arb file in the arb-dir (e.g., app_fr.arb).
• Translate all keys present in the template .arb file.
• Add the new Locale to the supportedLocales list in MaterialApp.
• Run validator -> Execute flutter gen-l10n to verify ARB syntax and regenerate AppLocalizations.
• Review errors -> Fix any missing placeholders or malformed plural/select statements.
• If targeting iOS, complete the "Configuring iOS App Bundle" workflow.

Task Progress: Configuring iOS App Bundle

Flutter handles runtime localization, but iOS requires bundle-level configuration for the App Store and system settings.

• Open ios/Runner.xcodeproj in Xcode.
• Select the Runner project in the Project Navigator.
• Navigate to the Info tab.
• Under the Localizations section, click the + button.
• Add the newly supported languages/regions.
• Run validator -> Build the iOS app to ensure project.pbxproj is correctly updated.

Troubleshooting & Gotchas

Missing Localizations Ancestor

Widgets like TextField and CupertinoTabBar require a Localizations ancestor with specific delegates (MaterialLocalizations or CupertinoLocalizations).

Error: No MaterialLocalizations found. or CupertinoTabBar requires a Localizations parent...
Fix: Ensure the widget is a descendant of MaterialApp/CupertinoApp. If building a standalone widget tree (e.g., in tests or a custom WidgetsApp), wrap the widget in a Localizations widget:

Localizations(
  locale: const Locale('en', 'US'),
  delegates: const [
    DefaultWidgetsLocalizations.delegate,
    DefaultMaterialLocalizations.delegate, // Required for TextField
    DefaultCupertinoLocalizations.delegate, // Required for CupertinoTabBar
  ],
  child: child,
)

Advanced Locale Definition

If supporting languages with multiple scripts (e.g., Chinese), use Locale.fromSubtags to explicitly define the scriptCode and countryCode to prevent Flutter from resolving to an unexpected variant.

supportedLocales: const [
  Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hans', countryCode: 'CN'),
  Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant', countryCode: 'TW'),
]