Countries if do trading or exchange for something they always use fiat currency. Most of the current use is dollar it is widely acceptable in all countries. Like my country want to buy oil in the middle east. The currency that will be use is $ against their oil. If you mean that bitcoin will be use for the purchase between two countries then its not possible yet.
Its quite the same in my country, if our country make trading to other countries they particularly use dollars as mean currency. But we did not know yet if bitcoin will be legalize in many countries it might be able for them to use bitcoin as trading payment between countries.