You know English better than Czech! Why was this not accepted as a corect answer? And You know more English than Czech is a correct answer??? For real?
If you wanted to say that you knew the Czech language well, ypu could also use the noun for Czech (language), which is "čeština".
The word "česky" here is not a noun but an adverb.
I think you speak english better than czech should be accepted. Lip comes from lepsi.
You speak English better than Czech is the proposed answer, at least presently.