Say English wins out, even if just for a while. Even if someone technically speaks English there are so many sub-cultural dialects and sets of slang. Ebonics, the Queen’s English, Irish brogue, New York Cab driver, etc. Hell, even the Marine’s have their own dialect. I would give you a sample but grunts and pointing don’t translate to the written word well and, beside, I have no poop to fling so my accent would be terrible.

These slang dialects can be so different from each other that it can damn near impossible to communicate between groups without loosing much of the nuance and meaning of the words.

As long as there is a human drive to maintain an “us” and a “them,” which barring an alien attack I don’t think will change, people will introduce novel features into how they talk to help tell the difference between the two. The distance between these groups geographically, socio-economically, etc will probably correlate to the difficulty in communication.