I guess these lines of code (includes/sef.php) need some modification?
//multilingual code url support
if( $mosConfig_sef && ($mosConfig_mbf_content || $mosConfig_multilingual_support) && $string!='index.php' && !eregi("^(([^:/?#]+):)",$string) && !strcasecmp(substr($string,0,9),'index.php') && !eregi('lang=', $string) ) {
if (strpos($string, '#')===false){
$string.= '&lang=' . $iso_client_lang;
} else {
$string = substr($string,0,strpos($string, '#')) . '&lang=' . $iso_client_lang . (strpos($string, '#')===false ? '' : substr($string,strpos($string, '#'),strlen($string)));
}
}