Flutter ローカライズ完全ガイド

import 'package:flutter/material.dart';
import 'package:easy_localization/easy_localization.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await EasyLocalization.ensureInitialized();

  runApp(
    EasyLocalization(
      supportedLocales: [
        Locale('en'),
        Locale('ar'),
        Locale('ja'),
        Locale('de'),
      ],
      path: 'assets/translations',  // Path to your ARB/JSON files
      fallbackLocale: Locale('en'),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: context.localizationDelegates,
      supportedLocales: context.supportedLocales,
      locale: context.locale,
      home: HomePage(),
    );
  }
}

import 'package:easy_localization/easy_localization.dart';
import 'package:flutter/material.dart';

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('appTitle'.tr()),   // Simple key
      ),
      body: Column(
        children: [
          // String with variable interpolation
          Text('greeting'.tr(args: ['Alice'])),

          // Named arguments
          Text('greeting'.tr(namedArgs: {'name': 'Alice'})),

          // Plural form
          Text('itemCount'.plural(3)),
        ],
      ),
    );
  }
}

// English: 3 useful forms (=0, =1, other)
"itemCount": "{count, plural, =0{No items} =1{1 item} other{{count} items}}"

// Arabic: 6 forms (=0, =1, =2, few, many, other)
"itemCount": "{count, plural, =0{لا عناصر} =1{عنصر واحد} =2{عنصران} few{{count} عناصر} many{{count} عنصرًا} other{{count} عنصر}}"

// Japanese: 1 form (other)
"itemCount": "{count, plural, other{{count}個のアイテム}}"

// German: 2 forms (=1, other)
"itemCount": "{count, plural, =1{1 Artikel} other{{count} Artikel}}"

// Russian: 3 forms (one, few, many)
"itemCount": "{count, plural, =0{Нет товаров} one{{count} товар} few{{count} товара} many{{count} товаров} other{{count} товаров}}"

// Simple variable
"welcome": "Welcome, {name}!"

// Multiple variables
"orderStatus": "Order #{orderId} — {status}"

// Variable in plural context
"unreadMessages": "{count, plural, =0{No unread messages} =1{1 unread message from {sender}} other{{count} unread messages}}"

// In widgets:
Text('welcome'.tr(namedArgs: {'name': userName}));
Text('orderStatus'.tr(namedArgs: {'orderId': '1234', 'status': 'Shipped'}));
Text('unreadMessages'.plural(count, namedArgs: {'sender': senderName}));

import 'package:flutter/material.dart';

class AdaptiveLayout extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final isRtl = Directionality.of(context) == TextDirection.rtl;

    return Scaffold(
      body: Row(
        children: [
          // Use start/end instead of left/right
          Expanded(
            child: Padding(
              padding: EdgeInsetsDirectional.only(
                start: 16.0,  // Leading edge (left in LTR, right in RTL)
                end: 8.0,     // Trailing edge
              ),
              child: Text('content'.tr()),
            ),
          ),
          // Flip icons for RTL
          Icon(
            isRtl ? Icons.arrow_back : Icons.arrow_forward,
          ),
        ],
      ),
    );
  }
}

// Use Directional widgets for RTL-aware layouts
EdgeInsetsDirectional.only(start: 16, end: 8)   // Not EdgeInsets.only(left: 16, right: 8)
AlignmentDirectional.centerStart                  // Not Alignment.centerLeft
BorderRadiusDirectional.only(topStart: Radius.circular(8))

// Automatically mirrors when locale changes to Arabic, Hebrew, etc.
// No conditional logic needed — Flutter handles text direction from locale

import 'package:locale_chain/locale_chain.dart';
import 'package:locale_chain/easy_localization.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await EasyLocalization.ensureInitialized();
  LocaleChain.configure();  // Enable smart fallback chains

  runApp(
    EasyLocalization(
      supportedLocales: [
        Locale('en'),
        Locale('pt', 'BR'),
        Locale('pt', 'PT'),
        Locale('es'),
        Locale('es', 'MX'),
      ],
      path: 'assets/translations',
      assetLoader: LocaleChainAssetLoader(  // Swap in the chain loader
        baseLoader: RootBundleAssetLoader(),
      ),
      fallbackLocale: Locale('en'),
      child: MyApp(),
    ),
  );
}

// Result: pt-BR user sees pt-PT translations when pt-BR keys are missing,
// instead of falling back directly to English.

// Override specific chains while keeping built-in defaults
LocaleChain.configure(
  fallbacks: {
    'pt-BR': ['pt-PT', 'pt'],      // pt-BR → pt-PT → pt → en
    'es-MX': ['es-419', 'es'],     // es-MX → es-419 → es → en
    'fr-CA': ['fr'],               // fr-CA → fr → en
  },
);

// Or replace all defaults with your own chains
LocaleChain.configure(
  fallbacks: {
    'zh-Hant-HK': ['zh-Hant-TW', 'zh-Hans'],
    'pt-BR': ['pt-PT', 'pt'],
  },
  mergeDefaults: false,  // Only your chains, no built-in defaults
);

# In your IDE, ask your AI assistant:
> Translate assets/translations/en.arb to Arabic, Japanese, and German

✓ ar.arb created (1.4s)
✓ ja.arb created (1.2s)
✓ de.arb created (1.1s)

# Or use the CLI in CI/CD:
npx i18n-agent translate assets/translations/en.arb --lang ar,ja,de

my_flutter_app/
├── assets/
│   └── translations/
│       ├── en.arb          # Source language (English)
│       ├── ar.arb          # Arabic (with 6 plural forms)
│       ├── de.arb          # German
│       ├── ja.arb          # Japanese
│       ├── pt-BR.arb       # Brazilian Portuguese
│       └── pt-PT.arb       # European Portuguese
├── lib/
│   ├── main.dart           # App entry with EasyLocalization
│   ├── app.dart            # MaterialApp with locale delegates
│   └── widgets/
│       └── language_switcher.dart
├── pubspec.yaml            # Dependencies
└── analysis_options.yaml