Internationalizing Flutter apps

Contents

Introduction to localizations in Flutter
Advanced topics for further customization
How internationalization in Flutter works
Alternative internationalization workflows
- An alternative class for the app’s localized resources
- Using the Dart intl tools

If your app might be deployed to users who speak another language then you’ll need to internationalize it. That means you need to write the app in a way that makes it possible to localize values like text and layouts for each language or locale that the app supports. Flutter provides widgets and classes that help with internationalization and the Flutter libraries themselves are internationalized.

This page covers concepts and workflows necessary to localize a Flutter application using the MaterialApp and CupertinoApp classes, as most apps are written that way. However, applications written using the lower level WidgetsApp class can also be internationalized using the same classes and logic.

Introduction to localizations in Flutter

This section provides a tutorial on how to internationalize a Flutter application, along with any additional setup that a target platform might require.

Setting up an internationalized app: the Flutter_localizations package

By default, Flutter only provides US English localizations. To add support for other languages, an application must specify additional MaterialApp (or CupertinoApp) properties, and include a package called flutter_localizations. As of November 2020, this package supports 78 languages.

To use flutter_localizations, add the package as a dependency to your pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations: # Add this line
    sdk: flutter         # Add this line

Next, run pub get packages, then import the flutter_localizations library and specify localizationsDelegates and supportedLocales for MaterialApp:

import 'package:flutter_localizations/flutter_localizations.dart';

return const MaterialApp(
  title: 'Localizations Sample App',
  localizationsDelegates: [
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
    GlobalCupertinoLocalizations.delegate,
  ],
  supportedLocales: [
    Locale('en', ''), // English, no country code
    Locale('es', ''), // Spanish, no country code
  ],
  home: MyHomePage(),
);

After introducing the flutter_localizations package and adding the code above, the Material and Cupertino packages should now be correctly localized in one of the 78 supported locales. Widgets should be adapted to the localized messages, along with correct left-to-right and right-to-left layout.

Try switching the target platform’s locale to Spanish (es) and notice that the messages should be localized.

Apps based on WidgetsApp are similar except that the GlobalMaterialLocalizations.delegate isn’t needed.

The full Locale.fromSubtags constructor is preferred as it supports scriptCode, though the Locale default constructor is still fully valid.

The elements of the localizationsDelegates list are factories that produce collections of localized values. GlobalMaterialLocalizations.delegate provides localized strings and other values for the Material Components library. GlobalWidgetsLocalizations.delegate defines the default text direction, either left-to-right or right-to-left, for the widgets library.

More information about these app properties, the types they depend on, and how internationalized Flutter apps are typically structured, can be found below.

Adding your own localized messages

Once the flutter_localizations package is added, use the following instructions to add localized text to your application.

Add the intl package to the pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: ^0.17.0 # Add this line

Also, in the pubspec.yaml file, enable the generate flag. This is added to the section of the pubspec that is specific to Flutter, and usually comes later in the pubspec file.
```
# The following section is specific to Flutter.
flutter:
  generate: true # Add this line
```
Add a new yaml file to the root directory of the Flutter project called l10n.yaml with the following content:
```
arb-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
```
This file configures the localization tool; in this example, the input files are located in ${FLUTTER_PROJECT}/lib/l10n, the app_en.arb file provides the template, and the generated localizations are placed in the app_localizations.dart file.

In ${FLUTTER_PROJECT}/lib/l10n, add the app_en.arb template file. For example:

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

Next, add an app_es.arb file in the same directory for Spanish translation of the same message:
```
{
    "helloWorld": "¡Hola Mundo!"
}
```
Now, run your app so that codegen takes place. You should see generated files in ${FLUTTER_PROJECT}/.dart_tool/flutter_gen/gen_l10n.

Add the import statement on app_localizations.dart and AppLocalizations.delegate in your call to the constructor for MaterialApp.

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

return const MaterialApp(
  title: 'Localizations Sample App',
  localizationsDelegates: [
    AppLocalizations.delegate, // Add this line
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
    GlobalCupertinoLocalizations.delegate,
  ],
  supportedLocales: [
    Locale('en', ''), // English, no country code
    Locale('es', ''), // Spanish, no country code
  ],
  home: MyHomePage(),
);

Use AppLocalizations anywhere in your app. Here, the translated message is used in a Text widget.
```
Text(AppLocalizations.of(context)!.helloWorld);
```
You can also use the generated localizationsDelegates and supportedLocales list instead of providing them manually.
```
const MaterialApp(
  title: 'Localizations Sample App',
  localizationsDelegates: AppLocalizations.localizationsDelegates,
  supportedLocales: AppLocalizations.supportedLocales,
);
```
This code generates a Text widget that displays “Hello World!” if the target device’s locale is set to English, and “¡Hola Mundo!” if the target device’s locale is set to Spanish. In the arb files, the key of each entry is used as the method name of the getter, while the value of that entry contains the localized message.

To see a sample Flutter app using this tool, please see gen_l10n_example.

To localize your device app description, you can pass in the localized string into MaterialApp.onGenerateTitle:

return MaterialApp(
  onGenerateTitle: (BuildContext context) =>
      DemoLocalizations.of(context).title,

For more information about the localization tool, such as dealing with DateTime and handling plurals, see the Internationalization User’s Guide.

Localizing for iOS: Updating the iOS app bundle

iOS applications define key application metadata, including supported locales, in an Info.plist file that is built into the application bundle. To configure the locales supported by your app, use the following instructions:

Open your project’s ios/Runner.xcworkspace Xcode file.
In the Project Navigator, open the Info.plist file under the Runner project’s Runner folder.
Select the Information Property List item. Then select Add Item from the Editor menu, and select Localizations from the pop-up menu.
Select and expand the newly-created Localizations item. For each locale your application supports, add a new item and select the locale you wish to add from the pop-up menu in the Value field. This list should be consistent with the languages listed in the supportedLocales parameter.
Once all supported locales have been added, save the file.

Advanced topics for further customization

This section covers additional ways to customize a localized Flutter application.

Advanced locale definition

Some languages with multiple variants require more than just a language code to properly differentiate.

For example, fully differentiating all variants of Chinese requires specifying the language code, script code, and country code. This is due to the existence of simplified and traditional script, as well as regional differences in the way characters are written within the same script type.

In order to fully express every variant of Chinese for the country codes CN, TW, and HK, the list of supported locales should include:

supportedLocales: [
  Locale.fromSubtags(languageCode: 'zh'), // generic Chinese 'zh'
  Locale.fromSubtags(
      languageCode: 'zh',
      scriptCode: 'Hans'), // generic simplified Chinese 'zh_Hans'
  Locale.fromSubtags(
      languageCode: 'zh',
      scriptCode: 'Hant'), // generic traditional Chinese 'zh_Hant'
  Locale.fromSubtags(
      languageCode: 'zh',
      scriptCode: 'Hans',
      countryCode: 'CN'), // 'zh_Hans_CN'
  Locale.fromSubtags(
      languageCode: 'zh',
      scriptCode: 'Hant',
      countryCode: 'TW'), // 'zh_Hant_TW'
  Locale.fromSubtags(
      languageCode: 'zh',
      scriptCode: 'Hant',
      countryCode: 'HK'), // 'zh_Hant_HK'
],

This explicit full definition ensures that your app can distinguish between and provide the fully nuanced localized content to all combinations of these country codes. If a user’s preferred locale is not specified, then the closest match is used instead, which likely contains differences to what the user expects. Flutter only resolves to locales defined in supportedLocales. Flutter provides scriptCode-differentiated localized content for commonly used languages. See Localizations for information on how the supported locales and the preferred locales are resolved.

Although Chinese is a primary example, other languages like French (fr_FR, fr_CA) should also be fully differentiated for more nuanced localization.

The Locale class identifies the user’s language. Mobile devices support setting the locale for all applications, usually using a system settings menu. Internationalized apps respond by displaying values that are locale-specific. For example, if the user switches the device’s locale from English to French, then a Text widget that originally displayed “Hello World” would be rebuilt with “Bonjour le monde”.

The Localizations widget defines the locale for its child and the localized resources that the child depends on. The WidgetsApp widget creates a Localizations widget and rebuilds it if the system’s locale changes.

You can always lookup an app’s current locale with Localizations.localeOf():

Locale myLocale = Localizations.localeOf(context);

Specifying the app’s supportedLocales parameter

Although the flutter_localizations library currently supports 78 languages and language variants, only English language translations are available by default. It’s up to the developer to decide exactly which languages to support.

The MaterialApp supportedLocales parameter limits locale changes. When the user changes the locale setting on their device, the app’s Localizations widget only follows suit if the new locale is a member of this list. If an exact match for the device locale isn’t found, then the first supported locale with a matching languageCode is used. If that fails, then the first element of the supportedLocales list is used.

An app that wants to use a different “locale resolution” method can provide a localeResolutionCallback. For example, to have your app unconditionally accept whatever locale the user selects:

MaterialApp(
  localeResolutionCallback: (
    Locale? locale,
    Iterable<Locale> supportedLocales,
  ) {
    return locale;
  },
);

How internationalization in Flutter works

This section covers the technical details of how localizations work in Flutter. If you’re planning on supporting your own set of localized messages, the following content would be helpful. Otherwise, you can skip this section.

Loading and retrieving localized values

The Localizations widget is used to load and lookup objects that contain collections of localized values. Apps refer to these objects with Localizations.of(context,type). If the device’s locale changes, the Localizations widget automatically loads values for the new locale and then rebuilds widgets that used it. This happens because Localizations works like an InheritedWidget. When a build function refers to an inherited widget, an implicit dependency on the inherited widget is created. When an inherited widget changes (when the Localizations widget’s locale changes), its dependent contexts are rebuilt.

Localized values are loaded by the Localizations widget’s list of LocalizationsDelegates. Each delegate must define an asynchronous load() method that produces an object that encapsulates a collection of localized values. Typically these objects define one method per localized value.

In a large app, different modules or packages might be bundled with their own localizations. That’s why the Localizations widget manages a table of objects, one per LocalizationsDelegate. To retrieve the object produced by one of the LocalizationsDelegate’s load methods, you specify a BuildContext and the object’s type.

For example, the localized strings for the Material Components widgets are defined by the MaterialLocalizations class. Instances of this class are created by a LocalizationDelegate provided by the MaterialApp class. They can be retrieved with Localizations.of():

Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);

This particular Localizations.of() expression is used frequently, so the MaterialLocalizations class provides a convenient shorthand:

static MaterialLocalizations of(BuildContext context) {
  return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
}

/// References to the localized values defined by MaterialLocalizations
/// are typically written like this:

tooltip: MaterialLocalizations.of(context).backButtonTooltip,

Defining a class for the app’s localized resources

Putting together an internationalized Flutter app usually starts with the class that encapsulates the app’s localized values. The example that follows is typical of such classes.

Complete source code for the intl_example for this app.

This example is based on the APIs and tools provided by the intl package. An alternative class for the app’s localized resources describes an example that doesn’t depend on the intl package.

The DemoLocalizations class defined below contains the app’s strings (just one for the example) translated into the locales that the app supports. It uses the initializeMessages() function generated by Dart’s intl package, Intl.message(), to look them up.

class DemoLocalizations {
  DemoLocalizations(this.localeName);

  static Future<DemoLocalizations> load(Locale locale) {
    final String name =
        locale.countryCode == null || locale.countryCode!.isEmpty
            ? locale.languageCode
            : locale.toString();
    final String localeName = Intl.canonicalizedLocale(name);

    return initializeMessages(localeName).then((_) {
      return DemoLocalizations(localeName);
    });
  }

  static DemoLocalizations of(BuildContext context) {
    return Localizations.of<DemoLocalizations>(context, DemoLocalizations)!;
  }

  final String localeName;

  String get title {
    return Intl.message(
      'Hello World',
      name: 'title',
      desc: 'Title for the Demo application',
      locale: localeName,
    );
  }
}

A class based on the intl package imports a generated message catalog that provides the initializeMessages() function and the per-locale backing store for Intl.message(). The message catalog is produced by an intl tool that analyzes the source code for classes that contain Intl.message() calls. In this case that would just be the DemoLocalizations class.

Adding support for a new language

An app that needs to support a language that’s not included in GlobalMaterialLocalizations has to do some extra work: it must provide about 70 translations (“localizations”) for words or phrases and the date patterns and symbols for the locale.

See the following for an example of how to add support for the Norwegian Nynorsk language.

A new GlobalMaterialLocalizations subclass defines the localizations that the Material library depends on. A new LocalizationsDelegate subclass, which serves as factory for the GlobalMaterialLocalizations subclass, must also be defined.

Here’s the source code for the complete add_language example, minus the actual Nynorsk translations.

The locale-specific GlobalMaterialLocalizations subclass is called NnMaterialLocalizations, and the LocalizationsDelegate subclass is _NnMaterialLocalizationsDelegate. The value of NnMaterialLocalizations.delegate is an instance of the delegate, and is all that’s needed by an app that uses these localizations.

The delegate class includes basic date and number format localizations. All of the other localizations are defined by String valued property getters in NnMaterialLocalizations, like this:

@override
String get moreButtonTooltip => r'More';

@override
String get aboutListTileTitleRaw => r'About $applicationName';

@override
String get alertDialogLabel => r'Alert';

These are the English translations, of course. To complete the job you need to change the return value of each getter to an appropriate Nynorsk string.

The getters return “raw” Dart strings that have an r prefix, like r'About $applicationName', because sometimes the strings contain variables with a $ prefix. The variables are expanded by parameterized localization methods:

@override
String get pageRowsInfoTitleRaw => r'$firstRow–$lastRow of $rowCount';

@override
String get pageRowsInfoTitleApproximateRaw =>
    r'$firstRow–$lastRow of about $rowCount';

The date patterns and symbols of the locale will also need to be specified. In the source code, the date patterns and symbols are defined like this:

const nnLocaleDatePatterns = {
  'd': 'd.',
  'E': 'ccc',
  'EEEE': 'cccc',
  'LLL': 'LLL',
  // ...
}

const nnDateSymbols = {
  'NAME': 'nn',
  'ERAS': <dynamic>[
    'f.Kr.',
    'e.Kr.',
  ],
  // ...
}

These will need to be modified for the locale to use the correct date formatting. Unfortunately, since the intl library does not share the same flexibility for number formatting, the formatting for an existing locale will have to be used as a substitute in _NnMaterialLocalizationsDelegate:

class _NnMaterialLocalizationsDelegate
    extends LocalizationsDelegate<MaterialLocalizations> {
  const _NnMaterialLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) => locale.languageCode == 'nn';

  @override
  Future<MaterialLocalizations> load(Locale locale) async {
    final String localeName = intl.Intl.canonicalizedLocale(locale.toString());

    // The locale (in this case `nn`) needs to be initialized into the custom
    // date symbols and patterns setup that Flutter uses.
    date_symbol_data_custom.initializeDateFormattingCustom(
      locale: localeName,
      patterns: nnLocaleDatePatterns,
      symbols: intl.DateSymbols.deserializeFromMap(nnDateSymbols),
    );

    return SynchronousFuture<MaterialLocalizations>(
      NnMaterialLocalizations(
        localeName: localeName,
        // The `intl` library's NumberFormat class is generated from CLDR data
        // (see https://github.com/dart-lang/intl/blob/master/lib/number_symbols_data.dart).
        // Unfortunately, there is no way to use a locale that isn't defined in
        // this map and the only way to work around this is to use a listed
        // locale's NumberFormat symbols. So, here we use the number formats
        // for 'en_US' instead.
        decimalFormat: intl.NumberFormat('#,##0.###', 'en_US'),
        twoDigitZeroPaddedFormat: intl.NumberFormat('00', 'en_US'),
        // DateFormat here will use the symbols and patterns provided in the
        // `date_symbol_data_custom.initializeDateFormattingCustom` call above.
        // However, an alternative is to simply use a supported locale's
        // DateFormat symbols, similar to NumberFormat above.
        fullYearFormat: intl.DateFormat('y', localeName),
        compactDateFormat: intl.DateFormat('yMd', localeName),
        shortDateFormat: intl.DateFormat('yMMMd', localeName),
        mediumDateFormat: intl.DateFormat('EEE, MMM d', localeName),
        longDateFormat: intl.DateFormat('EEEE, MMMM d, y', localeName),
        yearMonthFormat: intl.DateFormat('MMMM y', localeName),
        shortMonthDayFormat: intl.DateFormat('MMM d'),
      ),
    );
  }

  @override
  bool shouldReload(_NnMaterialLocalizationsDelegate old) => false;
}

For more information about localization strings, see the flutter_localizations README.

Once you’ve implemented your language-specific subclasses of GlobalMaterialLocalizations and LocalizationsDelegate, you just need to add the language and a delegate instance to your app. Here’s some code that sets the app’s language to Nynorsk and adds the NnMaterialLocalizations delegate instance to the app’s localizationsDelegates list:

const MaterialApp(
  localizationsDelegates: [
    GlobalWidgetsLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    NnMaterialLocalizations.delegate, // Add the newly created delegate
  ],
  supportedLocales: [
    Locale('en', 'US'),
    Locale('nn'),
  ],
  home: Home(),
),

Alternative internationalization workflows

This section describes different approaches to internationalize your Flutter application.

An alternative class for the app’s localized resources

The previous example was defined in terms of the Dart intl package. Developers can choose their own approach for managing localized values for the sake of simplicity or perhaps to integrate with a different i18n framework.

Complete source code for the minimal app.

In the below example, the DemoLocalizations class includes all of its translations directly in per language Maps.

class DemoLocalizations {
  DemoLocalizations(this.locale);

  final Locale locale;

  static DemoLocalizations of(BuildContext context) {
    return Localizations.of<DemoLocalizations>(context, DemoLocalizations)!;
  }

  static const _localizedValues = <String, Map<String, String>>{
    'en': {
      'title': 'Hello World',
    },
    'es': {
      'title': 'Hola Mundo',
    },
  };

  static List<String> languages ()=> _localizedValues.keys.toList();

  String get title {
    return _localizedValues[locale.languageCode]!['title']!;
  }
}

In the minimal app the DemoLocalizationsDelegate is slightly different. Its load method returns a SynchronousFuture because no asynchronous loading needs to take place.

class DemoLocalizationsDelegate
    extends LocalizationsDelegate<DemoLocalizations> {
  const DemoLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) => DemoLocalizations.languages().contains(locale.languageCode);


  @override
  Future<DemoLocalizations> load(Locale locale) {
    // Returning a SynchronousFuture here because an async "load" operation
    // isn't needed to produce an instance of DemoLocalizations.
    return SynchronousFuture<DemoLocalizations>(DemoLocalizations(locale));
  }

  @override
  bool shouldReload(DemoLocalizationsDelegate old) => false;
}

Using the Dart intl tools

Before building an API using the Dart intl package you’ll want to review the intl package’s documentation. Here’s a summary of the process for localizing an app that depends on the intl package.

The demo app depends on a generated source file called l10n/messages_all.dart, which defines all of the localizable strings used by the app.

Rebuilding l10n/messages_all.dart requires two steps.

With the app’s root directory as the current directory, generate l10n/intl_messages.arb from lib/main.dart:
```
$ flutter pub run intl_translation:extract_to_arb --output-dir=lib/l10n lib/main.dart
```
The intl_messages.arb file is a JSON format map with one entry for each Intl.message() function defined in main.dart. This file serves as a template for the English and Spanish translations, intl_en.arb and intl_es.arb. These translations are created by you, the developer.
With the app’s root directory as the current directory, generate intl_messages_<locale>.dart for each intl_<locale>.arb file and intl_messages_all.dart, which imports all of the messages files:
```
$ flutter pub run intl_translation:generate_from_arb \
    --output-dir=lib/l10n --no-use-deferred-loading \
    lib/main.dart lib/l10n/intl_*.arb
```
Windows does not support file name wildcarding. Instead, list the .arb files that were generated by the intl_translation:extract_to_arb command.
```
$ flutter pub run intl_translation:generate_from_arb \
    --output-dir=lib/l10n --no-use-deferred-loading \
    lib/main.dart \
    lib/l10n/intl_en.arb lib/l10n/intl_fr.arb lib/l10n/intl_messages.arb
```
The DemoLocalizations class uses the generated initializeMessages() function (defined in intl_messages_all.dart) to load the localized messages and Intl.message() to look them up.