Per @Johan’s comment on Slack:

So I’ve spent some more time looking at discussions around Commons/Wikidata issues, and while we’ve focused a lot on Wikidata lately, one thing I think we need to make much more clear (and this probably goes for Wikidata as well) is that you’ll be adding image captions in the app interface language. If you’re bilingual – or speak more than two languages – this is not obvious, and going through the process of starting adding them again in the app I couldn’t see any clear hints. (edited)