I am not sure it will happens. There is many countries with own currency in the word.every country want to keep their currency. I think a single currency in the word is impossible.
Every government of every country wants their economy to be higher than the other country the reason they are all working on it for the sake of their country and well, their pride. If there will be a single currency for the world, I think that is when the peace is really happening that even the currency and economy are merging into one.