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

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 change 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.