People talk about Bitcoin one day becoming a global currency but something we don't consider is a currency is anything that you can trade goods and services for.
Money (Fiat or cryptocurrency) is only worth what people value it at.
Bitcoin is valued at ~330AUD$ and is used globally.
Bitcoin already is a global currency. It is used around the world and is technically valued higher than any other currency in the modern world.
It's arguable as to whether its a global currency. Sure, it can be used globally but in not many places. It would be very difficult to travel the world on bitcoin or survive solely on it. I hope one day this will change however and it will take time but I do believe it could become the worlds first global currency.