I use Acrobat Reader on my iPad on a daily basis, and it was English until now. Today, I added Russian keyboard to my iPad's keyboards (I did NOT change the iPad's main language, it's still Hungarian, as it was before), and the Adobe Acrobat Reader app became Russian. I tried to remove the Russian keyboard, uninstall/reinstall the app, but it's still Russian. How can I set it back to English?
Sadly, there is not much we can do about this as it is the OS that controls the language you are using to run.
I suppose what happened here was that adding the Russian keyboard changed the order of the preferred languages list and removing the keyboard does not change it.
So for testing, you may change this default language to English which might change the list order and get English on top of the list again.