Is it correct to say: I know sufficient about the subject, or would it be correct to say I know enough about the subject? I heard people use the first version, but however, I am not a native speaker, to me is is sound somehow unnatural
I wouldn't say this use of "sufficient" is incorrect. It does also sound wrong to my American ears, but I have heard it used by some British English speakers. I would say "I know enough".
If I wanted to use "sufficient" in this context, it would have to be related to some attribute:
My knowledge of the language is sufficient (for me to get around comfortably in that country)
I am sufficiently fluent in the language (that I can get around comfortably in that country).