[ticket/13003] Add missing language keys

PHPBB3-13003
Merge pull request #2899 from Skouat/ticket/13003

[ticket/13003] Add missing language keys

* Skouat/ticket/13003:
  [ticket/13003] Add missing language keys
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13003] Add missing language keys