This PR depends on the proposed change in mkdocstrings/mkdocstrings#565, and I need to finish JP translations to call it somewhere ready for review.

Ready for review! The point open for bikeshedding is whether we are using locale or language as file / folder name.

Thanks so much! Could you take a look at the failing tests in CI? It seems locale is empty and therefore Jinja cannot find the language file.

Thanks so much! Could you take a look at the failing tests in CI? It seems locale is empty and therefore Jinja cannot find the language file.

Investigating, since even without mkdocstrings' core supplying locale it should automatically fallback to English.

Currently I found it problematic when the corresponding language file is not present. Looking for a solution :(

I pushed a few changes, I hope you don't mind. LGTM now, ready to merge, any objection?
Also, once we merged, we'll need to wire the logic from mkdocstrings' side right? By actually passing the locale? Would you like to work on this too?