I've done it this way and didn't learn much, I learned more by self-studying after I left. Thinking that I would just learn my target language just by being there was my biggest mistake.

Sure, if your target language isn't too far from your native one, learning it on the go probably works fine. But you aren't going to get from English to Chinese casually by picking up stuff though, you'll end up knowing a hundred words tops for your daily life and that's it.