It’s an excellent question. In the end I think it depends on the situation, which makes it hard to require of all librarians.
I’ve periodically wished I could read Russian due to certain materials in our collection, but the occasions are infrequent enough that actually learning Russian doesn’t seem like it would be worth the time I’d have to divert from other work-related things. (And what are the chances that Russian would be useful in my next position? Fairly small, I should think, though it’s certainly something that could be included on one’s resume.)
On the other hand, for someone on a public service desk in a linguistically diverse area, learning one of the non-English languages would be valuable.
I don’t think it’s a necessary requirement across the board (and just knowing an extra language wouldn’t necessarily help, if you studied French in college and then worked in a Polish immigrant community, for instance), but it’s definitely helpful at my public library.
I had grand plans when I started to take on Spanish, but then got sidetracked by schoolwork. Now sometime after finishing library school (August!) and Pip’s birth and adjustment to this big world (November!), I’d like to try again.