Per-user language takes priority
Every ahead ID user has a preferred language that can be defined by an admin in the User Management area. The onboarding invitation email now uses that language automatically, so invitations go out in the right language for each recipient.
Fallback language for users without a preference
For users who don't have a preferred language set, the email uses the language the admin picks in the "Send onboarding invitation" modal as the fallback language.
