I actually want to learn sign language. Everyone else takes Spanish, so it shouldn't be that hard to find an interpreter for you.. but sign language? I bet those are much harder to come across.
Nah, sign language people are everywhere. Anyone who wanted an "easy" grade in high school to fulfill their language requirements took sign language, and any school that doesn't want to deal with complaints almost has to have a few sign language interpreters on staff to travel around to various classes and interpret for deaf students. I would have no doubt that a sign language interpreter could easily be picked up. Hell, I learned quite a bit just through watching videos I rented from the library.
To the OP, I would suggest you do what you want to do, and be good at it. You'd be surprised, especially in the southern part of the country (like Louisiana, where the Cajuns and Creole's tend to be) how many people might speak French with only broken English. And when that day comes that you get a patient that only speaks French, you'll probably be one of the few people in the area that can actually help.
However, if you're looking for a language that would be USEFUL, that's going to depend on where you want to practice. For instance, the east coast has entire areas that speak mainly Russian, and Russian actually isn't that difficult of a language to pick up. Chinese has it's obvious benefits, as does Italian, Arabic, Farsi, whatever.
The point is that while Spanish will be the dominating foreign language in the medical setting, that also means that there will be an abundance of people who are available to translate. It's likely that several nurses and doctors in the hospital will even be native speakers. The people that are really going to need help are the ones who speak languages that don't come up as much. Hence, when they get sick, they have a much harder time finding someone who speaks their language than does a Spanish-speaking individual.
You can pick up Spanish over time. A lot of medical schools even teach medical Spanish as an elective. If you want to focus on a language now, I'd try to pick up proficiency in a language not many people try to carry with them, and I don't think French is a bad choice.