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.