Following on from I6fa966c2, fire the mw.uls.compact_language_links.open
hook with the $trigger parameter as context for clients to be able to
able to distinguish which treatment is visible to the user.
Supporting changes:
- Note that the hook is unstable next to its callsite
Bug: T280770
Change-Id: Ie4bbfea4dc5e36401df587550f466aeae5c8fdcc