Once we have strong AI, whatever that buzz word means, what then would be the usefulness of understanding slang?
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.