flutter/packages/flutter_localizations/lib/src/l10n/README.md

Material Library Localizations

The .arb files in this directory contain localized values (primarily strings) used by the material library. The localizations.dart file combines all of the localizations into a single Map that is linked with the rest of flutter_localizations package.

If you're looking for information about internationalizing Flutter apps in general, see the Internationalizing Flutter Apps tutorial.

Translations for one locale: .arb files

The Material library uses Application Resource Bundle files, which have a .arb extension, to store localized translations of messages, format strings, and other values. This format is also used by the Dart intl package and it is supported by the Google Translators Toolkit.

The material library only depends on a small subset of the ARB format. Each .arb file contains a single JSON table that maps from resource IDs to localized values.

Filenames contain the locale that the values have been translated for. For example material_de.arb contains German translations, and material_ar.arb contains Arabic translations. Files that contain regional translations have names that include the locale's regional suffix. For example material_en_GB.arb contains additional English translations that are specific to Great Britain.

There is one language-specific .arb file for each supported locale. If an additional file with a regional suffix is present, the regional localizations are automatically merged with the language-specific ones.

The JSON table's keys, called resource IDs, are valid Dart variable names. They correspond to methods from the MaterialLocalizations class. For example:

Widget build(BuildContext context) {
  return new FlatButton(
    child: new Text(
      MaterialLocalizations.of(context).cancelButtonLabel,
    ),
  );
}

This widget build method creates a button whose label is the local translation of "CANCEL" which is defined for the cancelButtonLabel resource ID.

Each of the language-specific .arb files contains an entry for cancelButtonLabel. They're all represented by the Map in the generated localizations.dart file. The Map is used by the MaterialLocalizations class.

material_en.arb Defines all of the resource IDs

All of the .arb files whose names do not include a regional suffix contain translations for the same set of resource IDs as material_en.arb.

For each resource ID defined for English in material_en.arb, there is an additional resource with an '@' prefix. These '@' resources are not used by the material library at run time, they just exist to inform translators about how the value will be used, and to inform the code generator about what code to write.

"cancelButtonLabel": "CANCEL",
"@cancelButtonLabel": {
  "description": "The label for cancel buttons and menu items.",
  "type": "text"
},

Values with Parameters, Plurals

A few of material translations contain $variable tokens. The material library replaces these tokens with values at run-time. For example:

"aboutListTileTitle": "About $applicationName",

The value for this resource ID is retrieved with a parameterized method instead of a simple getter:

MaterialLocalizations.of(context).aboutListTileTitle(yourAppTitle)

The names of the $variable tokens match the names of the MaterialLocalizations method parameters.

Plurals are handled similarly, with a lookup method that includes a quantity parameter. For example selectedRowCountTitle returns a string like "1 item selected" or "no items selected".

MaterialLocalizations.of(context).selectedRowCountTitle(yourRowCount)

Plural translations can be provided for several quantities: 0, 1, 2, "few", "many", "other". The variations are identified by a resource ID suffix which must be one of "Zero", "One", "Two", "Few", "Many", "Other". The "Other" variation is used when none of the other quantities apply. All plural resources must include a resource with the "Other" suffix. For example the English translations ('material_en.arb') for selectedRowCountTitle are:

"selectedRowCountTitleZero": "No items selected",
"selectedRowCountTitleOne": "1 item selected",
"selectedRowCountTitleOther": "$selectedRowCount items selected",

scriptCategory and timeOfDayFormat

The values of these resource IDs are not translations, they're keywords that help define an app's text theme and time picker layout respectively.

The value of timeOfDayFormat defines how a time picker displayed by showTimePicker() formats and lays out its time controls. The value of timeOfDayFormat must be a string that matches one of the formats defined by https://docs.flutter.io/flutter/material/TimeOfDayFormat-class.html. It is converted to an enum value because the material_en.arb file has this value labeled as "x-flutter-type": "icuShortTimePattern".

The value of scriptCategory is based on the Language categories reference section in the Material spec. The scriptCategory value is used when looking up the TextTheme, see the MaterialTextGeometry class.

Generated file localizations.dart: all of the localizations as a Map

If you look at the comment at the top of localizations.dart you'll see that it was manually generated using a dev/tools app called gen_localizations.

You can see what that script would generate by running this command:

dart dev/tools/gen_localizations.dart packages/flutter_localizations/lib/src/l10n material

The gen_localizations app just combines the contents of all of the .arb files into a single Map that has entries for each .arb file's locale. The MaterialLocalizations class implementation uses this Map to implement the methods that lookup localized resource values.

The gen_localizations app must be run by hand after .arb files have been updated. The app's first parameter is the path to this directory, the second is the file name prefix (the file name less the locale suffix) for the .arb files in this directory.

To in-place update the localizations.dart file using the default values, you can just run:

dart dev/tools/gen_localizations.dart --overwrite

Translations Status, Reporting Errors

The translations (the .arb files) in this directory are based on the English translations in material_en.arb. Google contributes translations for all the languages supported by this package. (Googlers, for more details see <go/flutter-l10n>.)

If you have feedback about the translations please file an issue on the Flutter github repo.

See Also

The Internationalizing Flutter Apps tutorial describes how to use the internationalization APIs in an ordinary Flutter app.

Application Resource Bundle covers the .arb file format used to store localized translations of messages, format strings, and other values.

The Dart intl package supports internationalization.