I spent my high school career, as well as my college years, studying Spanish. I got to be quite good at it, too. Unfortunately, I haven't used it since then and it's dwindled to practically nothing. *sigh*
I've wanted to learn Latin, too, but find it a bit difficult to work out all on my lonesome.
Start Japanese right away! It will take a good two or three years, if you study for at least two or three hours a day, before you can communicate in any meaningful way. There's no time to waste! And spend as much time as you can with Japanese people who don't speak good english.