[watson]

MacOS has this feature under Settings -> General -> Language & Region -> Preferred Languages. Here you can have an ordered list of languages you wish apps and websites (I _think_ only works for Safari) to use. I don't know how widespread this is in other OS'es or browsers.

[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).

[Aachen]

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

[detourdog]

I think the date might be later definitely after the adoption of unicode characters.