*Adapt language to user language for mail message

When creating a user in back-end and plg_user_joomla is set to send mail to user, the language used for the message sent by mail was the language used by the administrator creating the user and not the new user site language if that one was defined.
This patch will make it possible to use the language defined for the user instead. It will first try to use the user default site language and, if not set, then the new user default admin language. If no language is set it will use the administrator language used when creating the user as before the patch.

See http://forum.joomla.org/viewtopic.php?f=617&t=808993