Sounds fun to hear that " One World Currency" is Bitcoin but I must say that it's only be in crypto industries, many people and countries don't know about bitcoins and cryptocurrencies (as of now) later in future it will be called if fiat will be replace or bitcoins will be spread all over the world.
Gold is already a one-world currency and so are other precious metals. They can be bought and sold in every country if you bring them from another country, after some cumbersome paperwork. 
Now bitcoin has been used by different country users so in a way it is also a currency for the people who used it not just the world. Again the reason why different countries have different currencies is based on their economic standpoint and that is something that is a huge thing to take over with one currency. So I dont think that is happening anytime soon. Yes people there might use bitcoin but that is not the majority, fiat is and will be here for quite some time.
Thing might change if we see governments allowing crypto trade openly in exchanges of that country, as we know in many places it is not that open to all.