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 with a
Spanish translation of the same message:

    
{
    "helloWorld": "¡Hola Mundo!"
}