Hello.
I've written a very small patch that adds the following feature:

* when a new user logs in for the first time and a new user is created in the DB, the default language for this user is now written correctly.The value of the default language is taken from $CONFIG['locale_string'] (the "locale_string" value of the main configuration file)

The file patched is:
* /program/include/main.inc

the function patched are:
* rcmail_create_user
* rcmail_create_user
 
I've posted the patch on the sourceforge.net patch tracker too.
 
Best regards,
 
Andrea