I think that having Duolingo users do translations of Wikipedia articles would be a win-win for everyone. It would allow more advanced Duolingo users to translate more high-level sentances, and it would also greatly expand the amount of content on Wikipedia that is in languages other than English.
Maybe it would be possible for the people who run Duolingo to try to work something out with the people running Wikipedia?
Is there a wikipedia project group to add articles to duolingo then add them to wikipedia once created? Or even better is duolingo doing something along those lines automatically?
Check out Luis' TEDx talk (or the Duolingo video). Depending on the language you're learning, we have some Wikipedia content.