'They both deny their affair' is correct or you could say, 'They deny their affair.' They don't serve it here. I have limited knowledge of it. / I have limited knowledge about it. (DELETE the 'a') Candidates/ applicants must speak at least one foreign language for the job. = OK

'limited knowledge' is like 'little time' 'a limited knowledge' is like 'a little time' they are different in meaning/nuance; +/- We often say 'speak/be knowledgeable on a topic/subject' but not 'have knowledge on a topic/subject'.. it seems weird, somehow. 'of' seems best is this case, but 'I don't know much about that' is more common.

All are grammatically correct although I would say that these two sound a little unnatural although they are still correct: "I have a limited knowledge of it. / I have a limited knowledge about it." I would say "I have limited knowledge on it"