Suggestion: Music and Science
Just would like to suggest you to implement your site with music and maybe science knowledges like math, biology, history, philosophy etc... The engine is indeed very good for learning, I believe you would have success and so the people who would support you.
Try www.khanacademy.org. It's very similar to Duolingo. It's got video tutorials and exercises in everything from arithmetic to calculus, physics, biology, chemistry, economics, history, art history. They also have a computer science section where you can learn to write your own programs. It's also got a points system like Duolingo, and fun badges that you can earn.
And I just realized that the link doesn't work; you'll have to copy and paste it.
Second that. Khan Academy is absolutely fantastic for science / maths / economics.
@pazzi: Do you mean that you would like to see more math and science articles in the Translations section, or do you mean you want Duolingo to expand its educational mission beyond languages? Because, as two others commented, Khan Academy may be your answer for the latter.
My main point would be music, that could be considered as a language, and I don't know about any site with good engine for musical learning. Thanks for the replies!