That's how it was with me and sign language. I started learning it in 5th grade and got pretty good at it, but no one else was driven enough to learn more than what we had to learn for school (songs and stuff), so I didn't have anyone to talk to, so I forgot most of it.
It'd be cool to know French, but I don't think it'd be too useful. I don't know where/when would I use it.
I'm in my third year of french, and I am horrible at it. I can understand for the most part when it is spoken to me, or if I read it. But speaking/writing it myself is an insult to the language.