The primary language subtag for the locale.
This must not be null. It may be 'und', representing 'undefined'.
This is expected to be string registered in the IANA Language Subtag Registry with the type "language". The string specified must match the case of the string in the registry.
Language subtags that are deprecated in the registry and have a preferred
code are changed to their preferred code. For example,
const Locale('iw') are equal, and both have the
iw is a deprecated language subtag that was
replaced by the subtag
This must be a valid Unicode Language subtag as listed in Unicode CLDR supplemental data.
String get languageCode => _replaceDeprecatedLanguageSubtag(_languageCode);