The signature of WidgetsApp.localeResolutionCallback.
LocaleResolutionCallback is responsible for computing the locale of the app's
Localizations object when the app starts and when user changes the default
locale for the device.
locale is the device's locale when the app started, or the device
locale the user selected after the app was started. The
parameter is just the value of WidgetsApp.supportedLocales.
typedef LocaleResolutionCallback = Locale Function(Locale locale, Iterable<Locale> supportedLocales);