It turns out a lot of our translation fallback code was being reverted to the default locale, which resulted in a mishmash of translations. I’ve fixed it here and I think this should solve the problem:
https://github.com/discourse/discourse/commit/d1ebb9d0b5cb0dc960c8ea7d3cae18133b7b5d5c
(We should keep an eye on it though, there are a lot of fixes involved and I might have missed some edge cases.)