The short answer: it would make its citizens instantly wealthy beyond imagination.
Adopting bitcoin as a national currency would mean exchanging all of that nation’s currency for BTC, a process that, no matter how small the nation, would represent a colossal demand for the crypto-currency.
Additionally, as most nations today use the US dollar as a reserve currency, such a move would also represent the dumping of greenbacks, which would cause an equally massive downward pressure on USD.
Together, the USD/BTC cross would see complete collapse, dealing the already moribund dollar a mortal blow that would force all other nations to divest themselves of it, lest they be left with no value in reserves at all.
In selecting a new reserve currency, no nation could, of course, steer clear from the unavoidable political choice of picking the fastest appreciating, most secure and well proven currency the world has ever seen: bitcoin.
At that point, the average exchange rate at which the posited first nation managed to bitcoinise will pale in comparison to the rates other nations will pay in their frenzy to keep whatever wealth they have collected and said nation’s citizenry will have acquired a benefit which I needn’t expound on here.
As a final note I would add that having adopted bitcoin as a global currency would in one quick blow eradicate most of humanity’s suffering: no more hunger, no more war, no more illness, no more poverty. for these are all creatures of our degenerate monetary system, which in our age we have haplessly come to believe a necessary yoke, but which bitcoin will prove wrong.
That said, I have little doubt that no sooner is there talk of any such move by a nation, than will the US dispatch a “diplomatic” detachment to quash any such thinking, for no dollar dumping can ever be allowed (another topic I need not expound upon for the already abundant precedent to draw from).