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.