Its mostly seen as illegal to use undefined or self issued currency not minted by the government of that nation. I've heard of a few cases where people just turn out plain metal discs and have gotten into trouble because it was seen as a kind of counterfeiting operation that undermined the tax regime.
If you want to use currency outside of your own countries, its better to take up the established currency of a sovereign nation. In some cases you can hold a gold or silver coin that is still widely traded and accepted as valid. I realise some will argue its perfectly fine in theory but in practise it raises eyebrows and some alarm. Just carrying around large amounts of cash can cause forfeiture for example which is not exactly fair but common as an offfence because of the drugs trade unfortunately
If its going to paper money than there is no need to really use it since it can't be unregulated. The reason why bitcoin and all other crypto is so great is the trust system it has with blockchain backing it, paper money can't be backed by blockchain hence I think it will be "too unregulated", I need to have that regulation come from people and not government and if something is unregulated by governments than that's fine but if its unregulated by the people as well than that creates a problem. That is why there has been digital currencies before bitcoin but none actually took off like bitcoin did.