The actual only criteria of fiat money I learned in austrian high school was "government will accept taxes in that currency". We have quiet a long history of taxes here, ranging from cows or sacks of grain to gold to gold coins to gulden to reichsmark to schilling to euro. Probably a good measure of centuries more than the US (which basically got kickstarted with the UK system at the time). Funnily enough, the above definition was the only thing holding true through the centuries. Thing is, if your government accepts taxes in a certain currency (even if it's a foreign one, that's perfectly fine too) it creates a stability by itself which fosters the usability (call it user friendlyness if you like) of the currency.
In that sense, I'm fairly sure that unless you are personal friends with the president you cannot pay your taxes in Euro the same way you cannot pay them in ₿, Pesos or ¥ because the currency the US claims their taxes in is USD.
Please prove me wrong or gib a better definition if I'm wrong :)
Happy New Year everyone!