The interface language.
Does anybody else think that it would be neat if the interface language switched into the language you are learning after reaching a certain skill level? Perhaps after completing an "Interface skill-box".
Alternatively, your browser sends along your computer's chosen interface language with every page you load. Mine currently sends "Accept-Language:fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4", which indicates that I want my interfaces to be in French. Duolingo should just respect this. That way, when you think you're ready, you can change your computer's primary language and Duolingo will follow suit.
That's a good idea. I'm an English speaker learning Spanish. I had a go the other way around for extra practice and of course the instructions were in Spanish.
The interface changed to Spanish for me on the iPhone app after I changed the language setting for the whole phone to Spanish.