fix(i18n): translate notification messages to the recipient's language

Fixes #7241 

This list should cover everything:
 - [x] Likes
 - [x] Messageboard
 - [x] User validation by email (Some sites may allow selecting language already on registration)
 - [x] Groups:
  - [x] Add member
  - [x] Invite member
  - [x] Membership request
 - [x] Core
  - [x] Add user (admin action)
  - [x] Request new password
  - [x] Change password
  - [x] Reset password (admin action)
  - [x] Add friend
  - [x] Comment