Depends on how you define 'useful' I suppose. If you're going to take a lot of tests, then it would be most useful to know languages in the same family as English and tracing back to their roots so you could work out origins and stuff to help you. In that case German, Greek, and Latin would be most useful. French, Spanish, and Italian less so, but might help a bit.
If you are planning on using these other languages in travel it depends on where you'll travel. Work, same thing.
On the other hand, if you're planning on space travel you might want to pick up Klingon, Ancient, or Goa'uld.
