Allowing non-latin characters in registration

Try this (not tested, I hope I am not short circuiting it into endless loop):

add_filter('sanitize_user', 'non_strict_login', 10, 3);

function non_strict_login( $username, $raw_username, $strict ) {

    if( !$strict )
        return $username;

    return sanitize_user(stripslashes($raw_username), false);
}