'He didn't know more than 20 Czech words' isn't accepted?
Well, the original says "česky", not "českých slov".
Same meaning but not as literal I guess.
The hover popup translation for Český is simply "Czech". If Český translates "in Czech", I think it should say so, because to a native English speaker, Český appears to be an adjective, not an object of a preposition in this sentence
It is "Česky", not "Český"! It is an adverb, not an adjective.
And why not "He did not know Czech more than twenty words"?
That is an incorrect word order in English.