"I don't know French cinema well."
Translation:Je ne connais pas bien le cinéma français.
The definite articles le/la/les never become de in the negative sentences. Even the indefinite or partitive articles do not always switch to de after a negative; it happens only if they are the direct object in the sentence. In other words, in the negative sentences, you change an article to de if and only if it is an indefinite/partitive article AND it acts as an direct object. These two requirements must be fulfilled at the same time, no exceptions.