Right ? I am suprised that Facebook is actually the one leading in this UX: they clearly separate UX language ( singular ) and Languages which you don't need translation ( plural ).
Accept language header covers all that's needed. It's a ordered list of languages the user will understand in order of preference. You'd pick the first one as interface, don't translate anything that's in the list, and you can decide what to do with anything not in the list.
Sites that use ip of origin and just assume my language are such grating experiences.
Accept language is set at the browser level, where you usually want a single default with granular per-site (or even per-content) control.
There will be whole sites and apps that I want in a specific language that aren't my first preference. E.g. I might want my news and browser user interface in English but Google Maps in a local language, Netflix in the language of the content I watch the most etc.
Reality is just too complex for a single ordered list IMHO, having the default set to whatever heuristics that best matches the site, and give a very easy, prominent and persistent way to change the language is I think the best approach.
you still want a default for those website you haven't been yet. also you're conflating the user agent configuration with the trasmission protocol. the accept header easily accomodate your scenario with purely client side configuration, entirely transparent to the web site.
I really, really want to have a way to tell Youtube that if I enable subtitles and the content is either English or Portuguese, then the subtitles should be shown in the original language (either subtitles created by the author or auto-generated subtitles - sometimes I can't do audio), but if it's another language, it should be shown in English (again, either subtitles authored by a person, or auto-generated ones)
This extension can control subtitles so maybe there is hope that this or another extension will offer this kind of fine granularity
