I use both elements mentioned above - I reference that there's a certain dialect involved, and I try to integrate actual parts of the accent into the writing. It's best executed if you actually know how it's spoken and can say it aloud, and then reflect how it's said into your writing.

That said, I'm not particularly good at it - like all things, it's a matter of practice and skill with handling real world dialects to sound natural. I don't roleplay in other languages because a) I'm horrible with other languages and b) if I don't have a mastery of the language, I won't be able to properly connotate speech beyond "I suck speak".