Do nations really need to have a financial reserve in a particular currency of a nation? What are the implications of having reserves in multiple currencies? The dollar has been very stable because the US and its allies have always controlled the world economy. Currently, more nations like China and others have built their economy and they also want to control some segments of the world's economy.
I believe there are countries that have more than one currency as their federal reserve. Yuan, Pounds, Euros, Dollars are all federal reserve currencies. The dollar is only the most popular in this regard. Diversification isn't the issue here. The issue is if one of these currencies depreciates, it affects the country that is using it as a reserve.
What I find dreadful in all these is, a particular country can suffer from the negligence or bad government, bad monetary and fiscal policies of another country. Like how countries suffered because of the increased inflation rates in the U.S.
The point is diversification. Should any single currency fail, your portfolio will not entirely lose its value. USD is not stable anymore. Setting aside inflation -- more countries are becoming cautious about the U.S. governments ability to levy sanctions and forfeit assets against a particular country for their own internal political battles. Should you have USD and your country becomes barred from doing business with companies that the U.S. government has jurisdiction over, your currency loses a fair bit of value. The alternative isn't Yuan, it would be a decentralized asset. Some countries are turning to gold but IMO bitcoin would solve a lot of the issues that currency reserves have.
Looking from the perspective of counties that turn to gold and not Bitcoin, you can't blame them. You don't expect a country to keep its reserve in relatively new technology. Gold has been around forever and it is trusted. It can be also volatile like Bitcoin, but it's usually for the short term. Gold is still the best currency.