It's still money. Just because it's a foreign currency doesn't mean someone cannot accept it. It's still wealth.
Gold, stocks, real estate, bonds - that's all wealth. But you can't pay taxes with that, only with money you can.
That might be the definition of legal tender, but foreign currency is still money.
"Legal tender" is just the official status of money. Because Dollar is legal tender, you can pay taxes with them in the US. Yen is not legal tender in US, thus you can't pay taxes in the US. Ability to pay taxes is one important property of money, so Yen is not money in the US, although other features/properties of money for Yen are still given.
Being able to pay your taxes with something isn't the definition of something suddenly becoming money. Money doesn't suddenly become meaningless or worthless because you can't pay taxes with it. I can't pay US taxes with UK money but it's still money.
Some coiuntries even accept many other currencies from around the world. Some countries in Africa actually prefer dollars or euroes to their own fiat.
In those countries the US-Dollar is legal tender or defacto legal tender, so it becomes money there.
Not true. There are many countries in Africa all with different laws regarding what is legal tender. I doubt you can pay your taxes in US dollars in most of them either. People just choose to accept whatever they want because other currencies are stronger, but are you saying if the us dollar falls out of favor in the US and bitcoin became their defacto currency then it would become 'money' there even if you couldn't pay your taxes with it? It's still money regardless but you seem to be contradicting yourself here.