Sure, it's true that many things about good design are language agnostic. On the other hand, frameworks and languages can actually limit or enable what you can do, and that lack of familiarity with them has the potential to lead to mistakes.
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.
Even when I was back in college many years ago, only the first class taught a language. From that point forward the teacher of each class said we're using language X and suggested a book if you needed help learning.