Why not use crowdsourcing by asking a users native language to improve the quality of the lessons?
I've been doing Portugese and seeing as how there are so many mistakes I have to wonder, why don't you take advantage of all the Brazilian people on Duolingo to help fix the Portuguese lessons? Even if they are learning French or German you could ask them to rate/fix sentences. Maybe ask to approve/rate/fix one sentence per lesson they are working on (even if they are studying another language). Or better yet offer points to native speakers to fix/correct lessons and sentences in their native language. Instead of asking which language you speak when signing up for Duolingo, ask "What is your native language". I can't believe Duolingo doesn't do this.
I think that Duolingo is all about knowledge exchange and it would make sense for people to check sentences in their own language. However, I don't think that it is necessary to force people to do it as there are already a lot of people around here who check sentences out of interest or because they enjoy explaining things to others. If you feel that a particular language is under-represented then feel free to give a shout-out for natives to join that language forum.