AJAX - Umlaut Problem beim speichern von Formularfeldern

<script type='text/javascript'>  
<!--
var req;
function sendRequest(formdata) {
	try {
		req = window.XMLHttpRequest?new XMLHttpRequest(): new ActiveXObject('Microsoft.XMLHTTP');   
	} catch (e) {
		alert('kein ajax');  
	} 

	req.onreadystatechange = handleResponse;

	req.open('post', 'module/kontakt/func.php');  
	req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');  
	req.send(formdata); 
	return false;
}

function handleResponse() {
	if ((req.readyState == 4) && (req.status == 200)) { 
		alert(req.responseText);
	}
}
//-->
</script>

<form method='post' onsubmit=\"return sendRequest('wert1='+this.wert1.value+'&wert2='+this.wert2.value);\">  

...

$sql = sprintf("UPDATE `$dbtable` SET `WERT1` = '%s' WHERE `WERT` = '$wert' LIMIT 1 ; ", $_POST['wert1']);  
$sql = sprintf("UPDATE `$dbtable` SET `WERT2` = '%s' WHERE `WERT` = '$wert' LIMIT 1 ; ", $_POST['wert2']);