As a first iteration of the language variants support we're going to generate language variants for MCS content on the fly. In order to do that, #restbase would pass the `Acccept-Language` header to MCS when needed, and MCS should respect it and forward it back to #restbase when fetching #Parsoid content from #RESTBase before transforming it to mobile-sections or summaries.
In case requests to MW API are needed to get any content in the variant, we might need to make some library to transform `Accept-Language` into something MW understands, but I'm not sure that's needed.
NOTE: #RESTBase itself doesn't support forwarding the `Accept-Language` to MCS or returning correct parsoid HTML yet, it's blocked on Parsoid transform endpoint, so it won't be possible to properly test this just yet.