Why would a country do that if they can just issue a fiat currency?
Also, the viability of a currency is based on on the acceptance of a population. For instance in Eastern Europe during the Soviet era, the official currencies were only used for visible, above-board transactions. In the huge black market that flourished alongside the "normal" market, the only accepted currency was USD (then considered the safest, most fungible currency).
If people have confidence in a currency that's what they'll use. BTC knows no national boundaries as long as the internet exists.
What I meant was if a new country wanted to use BTC, how would they begin?
South Sudan would be a good example of a new country.