In my experience Accept-Language header is pretty unreliable. It tends to just be one of a few values. Between people never updating it, proxies stripping it, people concerned it will make them easier to fingerprint, etc.

Also, form an implementation standpoint, translation is a lot of work to get right and some companies may not do it for all languages so they may support the header but just not support the language choice you have. Yelp being a weird example where they apparently have the Japanese translation on another site.

Combine all that together and it's not usually worth the effort to do it unless you have specific legal requirements or high demand from users.