Promiňte, (ještě) neumím moc dobře česky - I am sorry, I don't speak Czech very well (yet).
Promiňte, učím se česky, ale ještě nemluvím. - I am sorry, I am learning Czech but I don't speak yet.
Já rozumím česky jen trochu/trošku - I understand Czech just a little bit.
Omlouvám se, ale (to) nevím - I am sorry, but I don't know (that).
To bohužel nevím - Unfortunately I don't know that.