[ticket/9975] Translate missing style error message

The error message about missing style data was not
translated

PHPBB3-9975
Merge pull request #1358 from MGaetan89/ticket/9975

[ticket/9975] Translate missing style error message
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/9975] Translate missing style error message

Conflicts:
	phpBB/includes/session.php