TO my opinion, only '我不會說中文' is correct. It means you are not able to speak Chinese well. '我不要中文' , word for word translation 'I don't want Chinese'. We usually don't say this. '我不知道說中文' , grammar inappropriate. We usually don't say this. '我不知道中文', word for word translation 'I don't know Chinese', sounds like you don't know there is a kind of language named Chinese in the world. '我不能說中文' , sounds like you are able to speak Chinese, but you are not allowed to do so. '我不可以說中文', same as '我不能說中文'.