How can Novak Djokovic speaks so many languages?

I think you’re asking the wrong question.

Historical Dictionary of Tennis:

One interesting aspect of the international quality of the sport is the fact that most of its top players are bilingual as a minimum and often multilingual. Serbian Novak Djoković, for example, upon winning a tournament in Italy, addressed the crowd in fluent Italian and then switched to near-perfect English for a television interview. Djoković, in addition  to his native Serbian, is also fluent in German and Croatian. In fact, virtually the only players who are not multilingual are the American and British native-English speakers.

Emphasis added. With extreme prejudice.

Djoka the polyglot:

Djoka en Français:

Djoka auf Deutsch:

Djoka en Español: