I agree with previous answers: it is totally acceptable to have characters, and their external POV voice, talking in slang.
But I would also add that I don't see a problem using slang even for an omniscient 3rd person POV, at all. If the language slang adds flavor to the narration, rhythm and worldview, I would use it. A 3rd person omniscient is a rhetorical artifact anyway, so it would be impossible to pretend pure objectivity in it. Being a voice in itself, it is always an ideal character telling the story to the reader, so it's not wrong if it has a personality.
The risk, however, is that this technique can subtract neutrality, or be too redundant or invasive. It must be used with taste and cleverness.
(I think I stepped a bit astray from the original question - I hope this helps anyway).