New language strings for buttons

New language strings for buttons like "Post Reply"

RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=43447
Ticket: http://tracker.phpbb.com/browse/PHPBB3-11328

Related RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=43583