Successful currencies meet three criteria:
(1) they are in widespread use as medium of exchange
(2) they serve as unit of account
(3) they are a stable store of value.
Most sovereign currencies fully meet all three criteria.
What about Bitcoin?
Bitcoin function as a currency maybe less than good. If bitcoin want to be used as a currency globally, it will have to do instantly.
The transaction confirmation of bitcoin is slow, but there will be lightning network which can improve greatly the speed of bitcoin transactions.