If everything else is equal, then obviously knowing the
current language I need will push that person ahead. IME, the language(s) a senior person knows is the least important part whether they are a good hire though.
My reasoning for this is that we are always learning new languages and using new frameworks. Why would I let a better person go when the language is probably going to change, or worst case they pick it up in a couple weeks just by looking at the existing code base? One case where I would deviate a bit is if I was hiring for a functional programming position. In that case I would prefer experience with some functional language, but that is not much different than wanting OO experience for a Java/C# position.