OK, OK, just one goofing off post before I get serious. This NEEDS to be said, just based on the title of the thread alone!
One language to rule them all, and one language to find them, one language to bring them into darkness and bind them!
OK, OK, I'll go find something to post on now. Oh, btw, it's currently not feasible to have a world language, but given a choice, with no reprecussions for either side, I'd have one world language. Imagine how many more people would be on this forum, for instance. How much more of the world we could interact with. Imagine if there were no need for translation, that everyone understood exactly what was meant, and no paraphrasing was required. Imagine how much more massive MMOs could be, how much more literature we'd have to read, how much more music we'd have to listen to. It would be something magnificent. One language gets my vote. One language to rule them all, and all that jazz!