Vary: Accept-Language is added to every cacheable response on the site

Problem/Motivation

ResponseSubscriber::onRespond() runs on every response of the whole site and unconditionally appends Accept-Language to the Vary header of anything implementing CacheableResponseInterface, which includes every ordinary HTML page. Browsers send highly variable Accept-Language values (en-US,en;q=0.9, fr-FR,fr;q=0.9,en;q=0.8, ...), so any reverse proxy or CDN honouring Vary fragments its cache of every page per browser language string, collapsing hit ratios for responses that do not vary by language at all. Only the module's own surfaces actually vary by that header.

Steps to reproduce

1. Install the module on any site behind a Vary-honouring proxy (or just inspect headers).
2. curl -skI https://example.test/user/login | grep -i vary contains Accept-Language, although that page does not vary by it.

Proposed resolution

Scope the Vary addition to the module's own surfaces.