Flutter Setup Localization Agent Skill

Internationalizing Flutter Applications

Contents

• Core Concepts
• Setup Workflow
• Implementation Workflow
• Advanced Formatting
• Examples

Core Concepts

Flutter handles internationalization (i18n) and localization (l10n) via the flutter_localizations and intl packages. The standard approach uses App Resource Bundle (.arb) files to define localized strings, which are then compiled into a generated AppLocalizations class for type-safe access within the widget tree.

Setup Workflow

Copy and track this checklist when initializing internationalization in a Flutter project:

• [ ] Task Progress
• [ ] 1. Add dependencies to pubspec.yaml.
• [ ] 2. Enable the generate flag.
• [ ] 3. Create the l10n.yaml configuration file.
• [ ] 4. Configure MaterialApp or CupertinoApp.

1. Add Dependencies

Add the required localization packages to the project. Execute the following commands in the terminal:

flutter pub add flutter_localizations --sdk=flutter
flutter pub add intl:any

Verify your pubspec.yaml includes the following under dependencies:

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

2. Enable Code Generation

Open pubspec.yaml and enable the generate flag within the flutter section to automate localization tasks:

flutter:
  generate: true

3. Create Configuration File

Create a new file named l10n.yaml in the root directory of the Flutter project. Define the input directory, template file, and output file:

arb-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
synthetic-package: true

4. Configure the App Entry Point

Import the generated localizations and the flutter_localizations library in your main.dart. Inject the delegates and supported locales into your MaterialApp or CupertinoApp.

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

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

Implementation Workflow

Follow this workflow when adding or modifying localized content.

1. Define ARB Files

• If creating NEW content: Add the base string to the template file (lib/l10n/app_en.arb). Include a description for context.
• If EDITING existing content: Locate the key in all supported .arb files and update the values.

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

Create corresponding files for other locales (e.g., app_es.arb):

{
  "helloWorld": "¡Hola Mundo!"
}

2. Generate Localization Classes

Run the following command to trigger code generation:

flutter pub get

*Feedback Loop:* Run validator -> review terminal output for ARB syntax errors -> fix missing commas or mismatched placeholders -> re-run flutter pub get.

3. Consume Localized Strings

Access the localized strings in your widget tree using AppLocalizations.of(context). Ensure the widget calling this is a descendant of MaterialApp.

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

Advanced Formatting

Use placeholders for dynamic data, plurals, and conditional selects.

Placeholders

Define parameters within curly braces and specify their type in the metadata object.

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

Plurals

Use the plural syntax to handle quantity-based string variations. The other case is mandatory.

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

Selects

Use the select syntax for conditional strings, such as gendered text.

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

Examples

Complete l10n.yaml

arb-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
synthetic-package: true
use-escaping: true

Complete Widget Implementation

import 'package:flutter/material.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';

class GreetingWidget extends StatelessWidget {
  final String userName;
  final int notificationCount;

  const GreetingWidget({
    super.key, 
    required this.userName, 
    required this.notificationCount,
  });

  @override
  Widget build(BuildContext context) {
    final l10n = AppLocalizations.of(context)!;

    return Column(
      children: [
        Text(l10n.hello(userName)),
        Text(l10n.nWombats(notificationCount)),
      ],
    );
  }
}