A knowledge of English. << This is morphologically COUNTABLE. Althought 2 knowledges of English is quite rarely heard. That means you take your expertise of something as a whole, similar to a certificate of English, a diploma of Chinese Literature, manifesting your level of professionalism in a sense. A "concept".
And when you mean to mean "a piece of knowledge" (yes, you could say that, like "a piece of information), it should consist of countless trivial terms, grammar rule 1, grammar rule 2, knowing how to spell the word "apple", etc. In this case, which is the most commonly used, it is then UNcountable.
Other than that, if you see "a knowlege" used unlike what I explained about in the first paragraph, but close to the "trivial" pieces one, that means "a kind of knowledge". Like when you say "a water", that means some specific kind of water probably in the lab, or "a people" referring to the people of the same nationality.