Currently, the HTTP Content-Language header returns the site's fixed content language. For a multilingual site like wikidata.org, this is useless at best, of not misleading.

At least on pages showing data items, the user interface language should be returned in the Content-Language header, to indicate the language actually used to render the page.

This would also help to solve the caching issues described in T43451.

See Also:
T43451: ULS causes pages to be cached with random user language