I built a similar site in 2009 focusing on Spanish/English exchange only (http://lenguajero.com). I learned a lot of lessons from the whole experience (like it's really hard to get people to pay for language learning on the web). We have around 13,000 members these days, but we're still small potatoes compared to livemocha.com or busuu.com.

I wish the verbling guys lots of luck with their new venture!