First of all, Bitcoin will be accepted by banks, and then some country can accept it as an official currency. Without doubt, most BTC users are in USA, but USA will not accept Bitcoin in nearest future. Most probably some small countries can accept Bitcoin, like Bahamas or Seychelles. And then we can wait for larger countries, like Canada or Sweden. 
No matters which country will take initiative the thing which will matter is the economic growth of that country and that country will lead the economy of world single handedly, and more chances of japan are being predicted that it will be considering it as to make national currency and many believes that japan will be more developed as compared to now. 
. All the countries will trust their own currency rather then depending on a currency whose price fluctuates a lot and even takes alot of time to confirm sometimes . So why to use such a currency when every country has its own well established currency