Teaching a kid two or more languages can be a challenge when people around you speak only one. The comments on this post – predominantly from immigrant parents raising their children in the US – are interesting, if for nothing else than the sheer diversity among them (one parent Korean-American, the other Spanish, for instance), and some funny anecdotes:

For example in Russian “Die” means give me. When we went to the park, the 3 year old ran towards a boy on a bike screaming “Die, Die, Die”. Where upon, the boys mother, shielded her son from my “crazy” daughter. I explained the situation and we had a good laugh.

Environment is definitely critical to retaining what you learn. I have friends who speak seven languages – all because all of those languages are spoken commonly around them.