What is difficult in doing this, is it the process that is difficult or the use of bitcoin as a national reserve the countries cant afford to take, i don't expect that they invest just like that without working on building a framework more suitable for them along side, bitcoin is not what could be done within a short period, but can be used as a tool in fixing the economic challenges that they may have both now and in the future, as everything points at the future rather than the present.
Most nations are already in huge debt, I mean all the nations that I know of are in big debt. It means, putting aside money to get some more bitcoin, is hard to explain to people. They already charge so much tax to cover the debt, or at least make less debt to be fair, and then not using that to pay for the debt but to use it for buying bitcoin is difficult.
This is why places like USA figured out an alternative, they are not spending tax money to buy, they just seize criminal peoples bitcoins to make sure they are used for reserve. That way nobody pays anything from their pocket, no tax goes there, and yet the nation still ends up with bunch of bitcoins as a reserve. This method is the best for any government out there, seize, and hold.