It's embarrassing, but like many Americans I'm monolingual; I'm only fluent in English.
I've taken classes in Japanese and Spanish over the years, but I'm far from conversational in either. With Japanese, I can recognize the odd word or phrase. With Spanish, I can read a paragraph and walk away with 40-60% comprehension, enough to get the basic idea but probably could not tell you exactly what was written. My comprehension when listening and ability to speak, however, is much worse.
The biggest reason that I never learned another language was simply that I didn't need to. Everyone in America speaks English! The immigrants that can't tend to lie low or bring their children to translate for them. Compared to Florence, where nearly everyone I meant could speak some English along with their Italian, there just isn't a need to use another language. I mean, it would have been useful to speak Spanish with customers back when I worked retail, but it had little effect on our bottom line that I couldn't. In Florence, though, there are hordes of American and English tourists with cash to spend, and if you can communicate clearly with them it probably makes a HUGE difference to the day's reciepts! Certainly, most of the Italians I spoke to would switch to English after I attempted more than one or two sentences in Italian, so I didn't get a lot of practice in for that language, either.
But what would I like to learn? Honestly, I do want to learn Japanese and Spanish. It's why I took those classes, after all. I guess I just don't want it badly enough to commit the time and energy it would take to master them. I always wanted to learn Latin, too, but again, not enough to really work at it.
I think that if I needed to learn a language - say, I moved to some European country for work - than I would. But if I don't have to, there's lots of other things I'd rather be doing.