== Background
From T188164#4324348:
> Per discussion on the Audiences-Services sync meeting, the initial step on the Page-Previews side could be to just explicitly set the Accept-Language header to the current wiki language code with no variant.
AIUI doing so will unblock Services rolling out language variant support for RESTBase and the Page Summary endpoint in particular. Moreover, landing language variant support for that endpoint will unblock {T111231} (blocked by {T198465} amongst others).
== AC
[] When Page Previews makes a request to the Page Summary endpoint:
- It includes the `Accept-Language` header
- The value of the `Accept-Language` header is the current wiki's language code **verbatim**, i.e. `$wgContentLanguage`.
== Notes
1. MediaWiki's language codes differ from ISO language codes in a number of different cases. Use the MediaWiki-provided language code (wgPreferredVariant) as that'll be handed to Parsoid/MediaWiki and that's what they understand.
See T198619#4387451, for additional detail about why this is important.