[oefrha]

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).

[happytoexplain]

It is not the accept-languages header - it affects the accept-languages header. Like the parent said, it is also used for apps (which includes your browser's UI).