I happily and successfully hire people who don't speak the language(s) we use, but use languages in the same general area. I've shied away from hiring people who have no experience in the area at all. For instance, I'll hire within (Java, C#, C++, Objective C) reasonably happily (and that's not a complete list), or (Python, Perl, Ruby, Lua, JS) fairly happily (with a special callout for JS to distinguish between "JS but only in the browser" and "actually knows JS", the former not necessarily counting as knowing how to program at all), but I hesitate to hire if you have no experience in the target cluster.

I think it's a big mistake to avoid hiring a Python programmer because they don't know Ruby, but it's one people seem happy to make, even in otherwise relatively enlightened organizations.