I'd say that in general (especially for algebraic geometry, but I really do mean in general) French is probably the way to go. Sure, there've been a lot of influential German-speaking mathematicians, but before 1820 or so they mostly wrote in Latin, and many of the important papers since then are available in translation.
In my experience, though, if you just want to read mathematical papers, it's not that hard (for a fairly well-read native English speaker) to pick up the basics of either French or German, since we have so many words derived from both. Of course, if you want to be able to write or speak the language, that's a different matter, but you can get surprisingly far with a dictionary and practice. (Note: This does not help with Bourbaki or Grothendieck.)
