[feature] Support any valid BCP 47 tag as a posting language

[VyrCossont]

Is your feature request related to a problem ?

As a client dev, I want my users to be able to post in whatever languages they use, and to be able to mark their posts as being in those languages. ActivityStreams specifies BCP 47 for language coding, so this could potentially be any language with a BCP 47 tag.

Describe the solution you'd like.

Current GtS only accepts bare ISO 639 language codes when posting, so you can post in en but not en-US, and zh but not zh-Hans or zh-Hant (this last is definitely a thing people might ask for, see mastodon/mastodon#18538 for background).

GtS should accept BCP 47 tags for post languages, and for an account's default language. BCP 47 tags should be canonicalized on receipt.

Describe alternatives you've considered.

Implementing an equivalent of the Mastodon API endpoint mastodon/mastodon#24443 to return a safelist to the client of all languages that are supported. We decided in chat that this was unnecessary.

Additional context.

For reference, mastodon/mastodon#23541 is the Mastodon equivalent of this feature request, and makes the same observation that AP/AS2 supports more than just ISO 639 codes.