Personally, I know Spanish fluently, good German, intermediate Italian and some French, Ancient Greek, and Arabic (I love me some languages ). Instead of seeing languages as a "research" thing, you should also consider how many fellowships and opportunities can open up for you when you have even an intermediate language ability. I'm thinking especially of opportunities in Germany and France, but also Italy to a lesser extent. You're generally casting a bigger net. Of course, if you're going to be a professor at a foreign university, you have to know the language rather fluently. But there are some opportunities to teach in English, more than ever actually.