I guess that we can already consider that Bitcoin is in fact a World currency. Bitcoin can be used pretty much anywhere. Anyone can use it, and there is no dependency on banks or other third parties. One can travel around the world just using Bitcoin, so it means that this is in fact a currency that goes far beyond any other fiat currency.

Now, you can say that legally, Bitcoin will never become an accepted and regulated international currency, but the fact is that Bitcoin is an alternative to the fiat system and as an alternative it already made its way to be accepted as a World currency anyway.