Yes, exactly... "Shall I help you with that?", "Shall I introduce you to my family?" etc. - polite/formal English.
It's not used much as a neutral future anymore, but can we really say this original function of shall is now completely gone from English? I.e. that it's wrong to use it today the way it was still used several decades ago? I can't make that call.
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.