Hmm, Spanish would definitely be the most practical...so, Spanish.
But, I would love to learn Arabic, or Hindi. I think Indo-Arabian languages are really pretty (Must be all the J-sounds). Plus I find alot of the throat sounds strangely sexy.
French is a pretty language too...Oh, and that one they speak in Beowulf! I remember Angelina Jolie's character speaking it, and just thinking how hot it was!