fix(messages): use recipient's language in the notification

Fixes #6902
Merge pull request #7189 from Elgg/message_notification_language

fix(messages): use recipient's language in the notification
fix(i18n): system cache now supports regional designators in language codes

Name of a cached language file can now have the following structure:
 - language code (2-3 lowercase alphabets)
 - underscore followed by regional designator (2 lowercase alphabets)

Fixes #7187
Merge pull request #7192 from juho-jaakkola/language_codes

System cache should support regional designators in language codes