Add a way for the user to select which languages they understand

[renchap]

Preliminary note

This issue is part of our "help wanted" issues. It represents something that we know we want to implement, and that is defined enough that a contributor can implement it in a way that it would get merged.

If you want to work on this, please add a comment saying so and ask any questions you need, then open a PR referencing this issue!

Description

This should be a similar user setting to the "filter timeline" languages that we have in Preferences > Other.

It would allow the user to pick a list of language they understand, which would disable the "translate" button for posts in this language. It may also be a good idea to show these languages on top of the language selection menu in the compose form.

Nice to have:

• It probably does not make sense to have "English (US)" and "English (UK)" as separate languages. I am not sure if this is the same for every language, maybe we should consider that if you pick en-US then all languages matching en-* are considered as spoken as well? Is this an issue with some languages?
• Should this be exposed publicly in the account AP object and federated, so UIs can use it for some nice features?

This would solve:

• Disable "Translate" option and/or add several spoken languages #23882

Could solve:

• Limit language switcher of compose to a few languages #28506
• Default to languages indicated by browser #27047
• Expose other users’ default/primary language #23067
• Pin user-selected content languages to the top of the editor language chooser #22726
• Cache recently used languages in post language picker #19510
• Limit languages to only those in use in drop down list #19911

Implementation notes

I am not sure we need to display a full list of languages with a checkbox for it. A nice picker that adds those to a list is probably a best UI, so only the ones you selected are shown

[tribela]

It probably does not make sense to have "English (US)" and "English (UK)" as separate languages. I am not sure if this is the same for every language, maybe we should consider that if you pick en-US then all languages matching en-* are considered as spoken as well? Is this an issue with some languages?

ko-KR and ko-KP has more differences than en-US/en-UK. Some sentences need translator to understand
BTW, Mastodon doesn't support ko-KP for now