Take Arjun Kapoor for example, who got a dialect coach to teach him Punjabi in Mubarakan, in which he plays a Sikh character. The role demands him not just to speak fluently in the regional language but in a specific dialect.
At the outset the filmmakers thought Arjun could perfectly master the Punjabi accent well as he understands the language but then a dialect coach was brought on board so that he's able to capture the subtle nuances rather well.
According to a source, "After just two days of training, the coach felt that the actor could speak the language with a proper accent, and didn't need any help anymore." Interesting as the actor essays a double role in the film.