We started teaching our son sign language before he started speaking, and now that he's speaking, my wife is teaching him both Spanish and English, so we're working on 3 languages in total.
As far as how it's going, he took to sign language VERY WELL. In fact, even though he can speak now, he still signs on a daily basis, to communicate simple things like hunger. It's interesting to hear him speak though, because some English words are easier for him than the Spanish counterpart, and vice-versa, so right now he's pretty much speaking in a mixture of the two, choosing to use the easier-to-pronounce word. (He says "agua" in place of "water", for example, but chooses the English "dog" in place of "el perro".)