Well for bitcoin to become a global reserve currency, i think the issue of volatility needs to addressed through broader adoption and integration, also a unified global regulatory frame work is essential to build the trust in bitcoin as a reserve asset.
for this to be possible, the government will have to set the price just like how they did it to Gold. but which government, because i don't think a country today will just accept what other country will say. that's why there are so many conflict today.
but there has to be unity where every leader of a country agrees. US can have their own but I think China will not follow Bitcoin as global reserves. if ever they follow, its would be them setting up rules and i don't think US will like that.
there will be tension before for BTC to be global reserve currency.