I just learned of Duolingo from a friend studying German. I've tried out the interface in English->Spanish study mode and I think it would be great for practicing sentence composition and translation - I haven't tried out the higher level exercises because my Spanish is pretty rusty, but the reviews seem good.
Anyways, they are continually adding new languages. And their twitter feed
https://twitter.com/duolingo/status/254268011756806144
says they will hopefully be adding Chinese in a few months.
So if you want more sentence/translation practice, check it out!