The infinitive form is "umět" but yeah, I think "which language do you know the best" is okay. On the other hand, I'm not a native English speaker so I'm not sure if that doesn't sound a bit weird. But when I think about it, I usually ask "what languages do you speak" and not "what languages do you know".
The root verb here is umíš (umět), and its basic meaning is "can" or "be able to" So the Czech sentence is not really about "speaking", nor about "knowing" in the sense of being familiar with something.
I think the best translation would be something like: "Which language are you best at?"