That actually kinda sucks then, if you can't chose. I hate when websites or apps chose what I want for me, because most of the time they get it wrong. Like IMDb translating all movie titles to German for me when I'm trying to find out the English name of a movie to watch it on Netflix -.-
I know the problem, but the solution is pretty easy: Look up the movie in the German Wikipedia - in the upper right corner you'll normally find the original title. Otherwise just click on "English" on the left side, so you'll get to the English article, named after - right, the original title. ;-) But if you want to be treated as a citizen of UK/USA/whatnot, you can use a proxy server. There are some available as add-ons for several browsers, I personally recommend "Zenmate" for Browsers based on the Chromium engine (such as Google Chrome or SRWare Iron). With this, you can choose what the internet thinks which nationality you're of.
I think it depends on what language you use to sign up. If I switch the mobile app to learning French from Spanish, I still get the interface in English. I think they're working on separating the interface from the language tree, Luis said they want to make it easier to switch between trees with different base languages.
Edit: Or maybe it depends on what language you have your device set to.