Along the lines of using foreign languages, I was reading a book set in the current South Carolina low country. One character was an elderly African American woman who supposedly spoke Gullah. Gullah is a real language, with strong influences from a number of African languages as well as English. This character's dialogue, however, was written phonetically in this horribly stereotypical "Mammy" language, which is NOT Gullah.
I stopped reading the book and I never do that.