Native language tag
Let the user check a box to select their native language in their profile. Then, if they did, display a little icon next to their messages in discussion section, or possibly only in the lessons of a skill tree involving their native language: if it is Spanish, let the tag show when they comment in the English>Spanish and Spanish>English skill trees, not in the English>French one, where their knowledge of Spanish is less relevant.
I think this would be useful in many subtle ways. What got me to write this is that i'm tired of warning people in discussions that "i am neither a native speaker of X, nor of Y, so i might be way off". Also, as a reader, i like to know whether a proposal for a "better translation", for example, comes from a person more competent in the source or target language. Finally, I met a bunch of people on duolingo spending significant amounts of time making "skill tree discussions" more insightful for other users, while not really getting anything out of it for themselves. I'd like to see these contributions conferred more "authority", at least in the eyes of the user who is willing to give more weight to the opinion of a native speaker.
Of course it should be left optional, no one has to let others know about who they are if they don't want to. Also, it might be better to stay out of it for people who truly master more than one language, as electing just one might be counterproductive for them with respect to the goal of this proposal. (and allowing the selection of more than one native language is probably a bad idea on the whole)
Thank you kristinemc, I'm very happy to hear that you guys have been thinking about this. It would help people like me, learning from their second language, a lot!
lieryan, I largely agree, I've met a number of very knowledgeable and helpful users on duolingo since i started too. There is already the up/down vote on each comment, as you noted yourself, some sort of aggregate-within-the-skill-tree of this could be used fairly easily to get a language-specific "rating". The downside is that when you start putting this type of things on in a community website, personal pride turns up, some begin to really really want to get a high score and strange behavior arises. Also, as you said yourself, if you do spend some time in the skill tree discussions, you learn very fast for yourself who's been of great help. Friend them, you'll remember them (they tend to be up you leader board anyhow :) ) It is slower for new users than seeing some sort of community medal, but at the same time it is much, much more relevant than what inevitably turns out to become a popularity vote of sorts...
To complement a native language tag, I would also like to see a way for users endorse/vote other users (esp. native speakers) who have been helpful and gives a lot of good insights about their language. This way, people (esp. native speakers) can be recognized for their contributions and proficiency even if they don't spend much time doing the exercises itself.
This would be similar to the discussion vote, but should be language-specific (an endorsement in German should not translate to an endorsement in Spanish, for example) and globally visible (at the moment you cannot see the total number of discussion votes people had received). If you, like me, likes visiting the sentence discussion pages while doing exercise, you'll start seeing a number of people who often replied with great insight to questions other users have on the sentence.