I have a feeling this is one of those places where ML will not be useful until we have strong AI. Certain grammatical errors are impossible to fix unless you understand the overall meaning of the text. Sometimes this meaning is embedded over many paragraphs. Errors involving incorrect word usage are unsolvable when words have more than one meaning and you don't comprehend the subject at hand.
Personaly I think the usefulness is already to be able to enterpret a concept encoded in slang as the same as the concept derived from a message encoded in a different dialect (or language).
I would never assume a machine spoke this language, only that they understood it. Machines should evolve into speaking succinctly as to not include unneccessary complexity in their messages as they would strive to be well-understood like all other persons do. I fail to see why we would want to produce slang-encoded messages, unless we want to mask the fact we are a machine.