You can say 한국어는 못 하지만 영어는 잘 해요

"한국어는 잘 못말해요 하지만 내 영어는 완벽하다" If I analyze it, 1. If the context is already language-related, as 한국어 in the beginning suggests here, we typically say the short form 하다 ("영어 할 줄 알아요?") because 말하다 is longer and sounds redundant in such cases. Also, 못 말해요 is wrong. We say 말 못해요 to mean "I can't say/speak it" (this is a long, tricky subject). 2. In Korean, 하지만(but) is never used in the middle of a clause, because we simply attach -지만 directly to the verb, as in 나는 키가 작지만 힘이 세다 = 나는 키가 작다. 하지만 힘이 세다 (하지만 sort of refers back to the previous sentence/clause). So you should either use -지만 or make it two separate sentences. 3. Using different endings (하다 vs 해요) in the same sentence is usually not a good idea. Better stick to one kind. Correcting these things, you can say any of these. - (나는) 한국어는 잘 못해요. 하지만 내 영어는 완벽해요. (colloquial feel) - 저는 한국어는 잘 못하지만 영어는 완벽해요. - 저는 한국어는 잘 못해도 영어는 완벽하게 해요. (more formal - it also uses consistent verb forms between the two clauses, which keeps the sentence in better balance)