Countries with limited population as well as having good technology development have already begun to circulate their own cryptocurrency. I read about Switzerland and Korea having their own cryptocurrency. This isn't much into usage now, can expect increased usage in the near future. Another important thing is the taxation, it'll hard to tax every transaction which is also a reason for the delay of cryptocurrencies usage within the countries.
If they circulated their own crypto currency, I don't think that is a problem to them to levy a tax on it. You said it yourself, "their own crypto currency", meaning it is not a decentralized crypto, it is centralized. I don't know whether they are really using it right now or still in progress or in testing but if they did, I don't think there will be a problem.
Many countries are looking into the idea of creating their own cryptocurrency but what they are doing is contradictory in nature, after all this market began with bitcoin and its main aim is decentralization which means that those coins that are created with the intention of competing against bitcoin will really have no chance against it because those coins are still going to have all the defects that you would find in the fiat system, which means that the governments knowing that they cannot stop bitcoin have decided to try to deceive people again just like what they did with gold.