Implement language fallback

Created by: mattflaschen

For instance, if the code requests a locale of de-CH, it should fallback to de, since currently there is no de-CH. This is an IETF language tag, an extension of ISO 639-1.

That way, you can just request any language without knowing what's implemented. And if a de-CH localization is added later, it will just work a little better, without the client code changing anything.

Fallback to English already works for when you request something like zh which isn't implemented at all.

You may be able to import the fallback configuration from MediaWiki. They have fallbacks configured (e.g. https://git.wikimedia.org/blob/mediawiki%2Fcore.git/d7a61d72231861755f0ccdaf80d7b4388cebd5df/languages%2Fmessages%2FMessagesMwl.php has a fallback to 'pt'), except when the fallback is to English.

Note that they are often circular, though ('pt' falls back to 'pt-br', which falls back to 'pt').