Would you learn Mandarin on Duolingo? If not, why not?
But if they add the user added language feature in October and include a way to add languages that don't use our letter system then it's quite possible it'll be on here is some form by 2014. Maybe I'm being a bit optimistic but the site is expanding.
Mandarin is a cool language, but it isn't top of my list.
1) I want to get to the end of my German tree and get to level 5 before I get serious about another language.
2) I'd want to start something that has more immediate use and ease of practice like French or Dutch.
3) If Duolingo comes out with a way to work with other languages and words represented by one symbol it will probably be very buggy at the beginning and I'll leave it up to the extremely passionate and knowledgeable to be the first intrepid explorers.