Sorry, I read it above wrong. It is much better to reply and correct me than just downvote, because then I get an e-mail and I can react. With a downvote I get nothing.
So yes, I was wrong. "I know xy" where xy is a language does not seem to be idiomatic English. If your experience is different please reply.
I agree. "Will I know Czech" does not seem a natural English phrase, but "Will I speak Czech", as suggested as the answer, is surely not correct either, because umět is more than just speak. "Will I have a good knowledge of", or "Will I be fluent in" are probably better phrases in English, but unlikely to be considered correct in Duolingo.