We want the new PCS endpoints (mobile-html, metadata, media, media-list, references) to actually vary their output for different language variants based on the correct accept-language header used.
This seems to be done already for the summary endpoint[1].
Example:
GET https://sr.wikipedia.org/api/rest_v1/page/mobile-html/Канада Accept: */* Cache-Control: no-cache Accept-Language: sr-el
should provide a different output than
GET https://sr.wikipedia.org/api/rest_v1/page/mobile-html/Канада Accept: */* Cache-Control: no-cache Accept-Language: sr-ec
[1] https://github.com/wikimedia/restbase/blob/master/v1/summary_new.yaml#L21