It is already a global currency because anyone can freely use it but the real problem is the issue about the legality in countries and this is the reason why bitcoin is being prevented from reaching its mass adoption and be used as a medium of payment or as a currency. But I understand why some countries dont want it as a currency it is because of the volatility of it and it is hard to use a currency that is not stable. And I think it will never be stable because it is being traded and people are just into it because of the profit that they can get from investing in bitcoin.