There has not been a country that officially approved the use of bitcoin, the main factor because they are still using fiat money for all transactions. Bitcoin will never replace a fiat money because bitcoin is decentralized and could not be in control.
Well there are already countries that recognizes and is in approval of btc. I've read that the philippine central bank already does this. Though i agree with you on the part that btc won't ever replace fiat. Possible even that without fiat bitcoin wouldn't survive. That is at thia point but i don't think there would be a scenario where in it would have to be a choice between the two. We can work better with both anyways
i do not think that there is any single country where the government has consider bitcoin as legal currency. i think still some countries are thinking about it but still they do not consider bitcoin as legal currency.