Content from the page content service doesn't match the language variant preference for users of wikis with language variants (zhwiki, srwiki, and others)
Enhance the content service to have the content returned be in the first available language variant of the user's preferred language variants. Preferred language variant lists would be communicated using the Accept-Language header in the standard format ( https://tools.ietf.org/html/rfc7231#section-5.3.5 ).
Utilize a method similar to core to process the Accept-Language header and pick the appropriate variant for the request:
Logic for parsing Accept-Language in core is WebRequest::getAcceptLang() (general header parsing, sorts by priority given) and LanguageConverter::getHeaderVariant() (iterates through language codes converting them to valid variants and picking one). Also LanguageConverter::validateVariant() which removes case sensitivity, remaps deprecated codes, converts zh-cn to zh-Hans-CN, etc.
Only variants that are valid for the domain requested should be returned. For example, requests to https://sr.wikipedia.org will only pick a Serbian variant even if the user has English with a higher rank in their Accept-Language header.