Noted incidentally at the bottom of T203095#4544592 was this:
There are some comments about accept-language header via ULS. Keep in mind this is disabled on Wikimedia Wikis (via $wgULSAnonCanChangeLanguage) and generally requires specialized varnish config to work. Have you checked with Traffic that this language workflow is workable.
Since we've never turned on $wgULSAnonCanChangeLanguage here before, I don't actually know what's involved in supporting it. From the brief text above, I roughly assume wikis that turns this on will probably emit a Vary: Accept-Language header for affected URLs, and that we'll probably need to normalize clients' Accept-Language according to rules similar to whatever ULS uses (e.g. pick first supported entry) for performance reasons. Someone will have to dig into this a bit and figure it out, and apparently there's not much time left to do so!