Ideally, countries should only be allowed to issue their own currency. It's not hard to see why a country being able to print another country's currency would pose a problem... With all money being digital and stored on thousands of distinct bank ledgers which basically don't have consensus, it would be very difficult to track with manual audits and with the current incentives in place. It would be trivial to hide these transactions under legitimate names as various forms of international payments.
Back in school we used to do things like "dare" people to do something or bet on a coin toss or something with the stakes being a snack or a desirable bit of stationery. I feel like everyone believes there is some clear boundary where things suddenly become "grown up" and "real". Do you remember when you became "grown up"? No? Nobody does. The secret is it never happens. The only difference is children have no "real" assets to play with.
What makes some derivative traded on Wall Street, like a "synthetic CDO", any more or less meaningful than alt coins?
And then stable coins. Fancy IOUs of fiat. Which might or might not have actual assets backing them. Which you might or might not be able to redeem. Say if Russian government had a billion in whatever stable coin. Could they redeem them and get real dollars transferred to some account they own?