Every browser I've used for a long time has supported Accept-Language with a preference list I can change. Are there any that don't?
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...
In regards to browser support, you are correct—however, changing preferences can be cumbersome.
Interestingly, there appears to be several browser add-ons that provide Accept-Language quick-switch capability in the browser UI, and that's more in line with what I had in mind with my original comment.