Djokovic's English is pretty fluent I'd say though like all players he speaks this weird sort of international English where you use the phrase 'for sure' too much and in a way that native speakers don't. 'He's a good player, he played aggressive, but I came out, for sure, and played aggressive as well'.
Nadal is the shocker. His English is stunningly bad for someone who has such an international job. Disclaimer, I'm rubbish at languages but actually my Spanish is better than his English.
Of course Fed speaks beautifully doesn't he, but then he would.