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.
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.
Besides, I think the very definition of what is money has changed or at least evolved with bitcoin.
No, Bitcoin does not change that. A power could define bitcoin as legal tender, but other features/properties of money are not given for bitcoin.