For example Japan back in 2017 accepted bitcoin as a "currency" which you can use in payments which seems to be the same definition as "legal tender", isn't it?
I think the situation is the same in
Germany, at least when it comes to paying taxes in case BTC is used as a means of payment.
There is a difference between "currency" and "legal tender".
A currency is anything which can be used as money, but a merchant is under no obligation to accept. In the US, for example, we recognize Canadian dollars as a currency, but it is not legal tender and you'll be very hard pressed to find anywhere outside a few dozen kilometers of the border which will accept CAD for goods or services. We recognize JPY, EUR, AUD, etc., as currencies, but none of them are legal tender.
Anything which is legal tender must be accepted as payment for goods, services, debt, etc. It is legally recognized by the courts or judicial system in that country. If I pay you in legal tender, then the debt is settled. Even if you refuse my payment, legally speaking I have paid my debt using legal tender and therefore owe you nothing.
Basically this is saying that every merchant, every business, every person, in El Salvador must accept bitcoin for their goods or services.