Bitcoin is most commonly known and used token in cryptocurrency. Suppose a country began to use Bitcoin instead of their currency. Then, what will happen?
First Bitcoin is not a token but a currency. The difference between a token and a currency is quite simple, while a token is a digital asset built on top of an existing Blockchain, Bitcoin as a cryptocurrency exists on its own Blockchain and is widely acceptable both as a medium of exchange and as a store of value. If countries begin to use Bitcoin as their sole currency they will have to deal with the volatility that comes it with. It's better we stick to our fiat currency and accept Bitcoin as an alternate currency and not the base currency.
Finally, if Bitcoin is the currency of a country, it will start a new economic age of that country. This will improve the economy of that country and some damage. So these positive and negative aspects of that country should be noticed equally.
Bitcoin will obviously improve the economy of any country if they have Bitcoin reserves aimed for a long term goal. But as a medium of exchange it won't improve their economy that much because they will spend more than the invested due to price fluctuations.