This is yet another thread that forums like this were made for. This is a really intriguing question. I’m not really sure there’s a good way to tell. Language is such a divider in itself…it is what helps keep people and cultures separated and defined. It would be interesting to see what would happen. I would guess that given enough time, people probably would end up with one language.
The world is so physically large though, we would have to be talking about an incredibly free world society…there would also have to be much less restrictions on movement…both institutionally and economically. People would have to be much free-er to get around and interact…without the hassle and restrictions of government travel and immigration agencies. And travel costs would have to drop dramatically (which of course a completely free society would have no problem accomplishing in a relatively short amount of time.) It would definitely take some time, but it would make sense that the language would go the way of money…for essentially the same reasons.