Well somebody along the way, be they parents or people in the schools, or friends, whomever, didn’t take it upon themselves to teach her how to speak correctly. Using slang is fine, as long as you know how to speak and write properly.
Children don’t naturally know how to speak, they copy those around them. When I’m around my friend’s children and they speak improperly, I correct them, so later on, they will know and can get hired somewhere. Apparently nobody at home or in a school drilled it into her the proper way to speak.
Makes me think of the old Steve Martin routine, "Can I mambo dogface to the banana patch."