Bitcoin will be a Global Currency if all countries are properly connected online or through internet. Sadly there are still countries where internet is still not available and if it's available signal is bad or expensive to get a signal. We all know that we cant transact using Bitcoin with out using internet.
Yes many are not connected in online because of internet connections especially in the poor area or the tribe they dont know how to use gadgets so bitcoin can not becoming global currency.
If 30-60% of the world population use the bitcoin, it can be regarded as a global currency. You can use it to buy a lot of things.