Banks in Germany started to allow people to buy and sell cryptocurrencies this January 2020. I think other countries will do the same soon, Japan might be another country with such services. In the beginning, someone has to start something so others can follow suit.
What are the other countries where anyone legally can buy/sell cryptos at commercial centralized banks?
I think you are a bit confused. The government/bank is not buying or selling crypto currencies. They are just allowing their clients to make crypto purchases online. If I remember, some banks at different countries were blocking transactions that involved crypto currencies. Where did you get this news from? Post a link over here so I can clarify.
Crypto currencies and banks don't go with each other. If banks started to sell crypto currencies, they will end up strictly regulating the currency which in turn will cause people to lose their financial freedom.