It's not about his skill at speaking the language, it's about understanding what is appropriate to say and what is not. Just because someone is technically skilled at something doesn't mean they know the best way to use it. That applies to everything including language and tennis.
It's not about the language, it's about the culture, Djokovic is the same. And i don't think he doesn't understand what is right to say and what isn't. If you understand the language you know what is appropriate and what isn't.