Many countries already use money internally such as the USD (outside the US) or Euro (outside the EU) for which they do not control the policy. Explicit agreements to use a common currency across nations and share control of the policy are relatively rare; no examples come to mind apart from the EU, and that hasn't always gone according to plan, as Greece can attest. But hard currency is still a fairly common basis for exchange between nation-states, and other countries' currencies are more likely to be adopted when they are governed by relatively immutable policies. Of course, if those policies change to be less immutable it can take time for the effects to manifest. The USD was relatively stable until recently, but other countries are probably reconsidering their dependence on it at this point given the increase in the supply over the past few years.

If Bitcoin does eventually become a common instrument of trade at this level it will fill the same niche currently occupied by gold and other precious metals.