Post language strings with country code are invalid according to BCP47

[TheEssem]

Steps to reproduce the problem

1. Create a post with the language set to a locale with a country code (e.g. zh-HK)
2. Obtain the JSON-LD of the post (e.g. using curl -H "Accept: application/json")
3. Note the keys of the contentMap object

Expected behaviour

The first key of the object should be a valid locale string in accordance with BCP47 (e.g. zh-HK)

Actual behaviour

The first key is in camel-case without a hyphen (e.g. zhHk)

Detailed description

Introduced by #27099. It seems like this is how languages are currently stored in the database, so some conversion is likely needed before sending a post to inboxes.

Mastodon instance

No response

Mastodon version

No response

Technical details

• Ruby version: 3.2.2
• Node.js version: 18.17.1

[ClearlyClaire]

This is most likely caused by app/lib/activitypub/adapter.rb, and new because of #27099 allowing for keys that would actually get transformed.

I'll see how to fix that.