In a video broadcast to Bitcoin 2021 President Nayib Bukele announced El Salvador’s partnership with digital wallet company, Strike, to build the country’s modern financial infrastructure using bitcoin technology.
I suspect this will be the same (in practice if not in theory). You can pay people in Bitcoin, or if they don't actually want Bitcoin the president's buddy is hyping an app that will let them get USD instead when you send your satoshis...
It looks like Japan did not, in fact, declare Bitcoin to be legal tender ("a form of money that courts of law are required to recognize as satisfactory payment for any monetary debt"). Rather, it imposed a regulatory regime on Bitcoin exchanges.
(That would be a nightmare - in my opinion, it would at least require some kind of official exchange rate between Bitcoin and yen.)