First of all, you dont need to edit core files.
Check this plugin:
http://wordpress.org/extend/plugins/wordpress-turkce/
This plugin letting “ŞşİıĞğÜüÖöÇç” (Turkish) characters in permalinks, usernames and blog names. You need to define those 2 variable for usernames and blog names:
define( 'WPTURKCE_USER', true );
define( 'WPTURKCE_BLOG', true );