well, i cant see it being a problem, personally. infact, i think it would be a good thing! i would feel happier and safer travelling anywhere in the world, just knowing that where ever i am, someone speaks the same language as me! not only that, you would be able to work anywhere in the world, there would be no reason why you couldnt buy a flat in tokyo, get a job, and live there!
on the other hand, as many people have already pointed out, identity is a main problem. which is why it has never happened. there are too many factors as to why this is the case, but thats just the way it is. language is as equally as important as culture, to destory one or the other, destroys a great deal of the culture (and many of things as well)
maybe we can only find a balance. i.e. learn english as a 2nd language making the local language priority.