I don't think there is an option for it yet, it just uses whatever your system language it set to, unless it isn't available. My phone is in Spanish yet the app in English, with the exception for "(x) minutes/days ago", which is Spanish funnily enough
My jerboa always displayed only numbers without time units (e.g. 3 instead of 3 mins or hours). I assumed that it's just a bug (and an extremely irritating one, since I never knew whether a post is 3 hours old and I can discuss under it, or is it 3 years old and it would be considered foul necromancy). Now I see that it might be simply the case that there are no translations of time units for my language yet (it's not as popular as English or Spanish so). And I think it would really help if it could be even set fixed to English if it can't be otherwise coordinated with the app, for it can sometimes be problematic.
I noticed the same thing after the last update. I would also prefer English as the interface language. Before the update everything was in English and now I have a mix of German and English. I already selected English as the interface language on the website but it didn't chance the ones in the app. In the app itself I haven't found any settings regarding this. I'm sure this is because my device language is German.