That's just the standard Accept-Language header, not unique to Safari or macOS at all. It's up to the server to interpret and respond to the header, and they mostly don't do what gp wants, i.e. they usually serve the translated language with a higher q rather than the original language even if it's in the list (and it's mostly ignored anyway in favor of geoip/manual region/language picker).

This has been OS-independent since the dawn of time (see the Accept-Language header specification), but sites rarely use it