PHP / XPath: Selecting namespaced elements

I have the following XML and I would like to select the xt:vars nodes with XPath in PHP.

<xt:locale xmlns:xt="http://www.example.net/xt">
    <xt:vars title="Invitation default" type="email" language="EN">
        <xt:param name="description">Default invitation mail added to event on creation.</xt:param>

    </xt:vars>
    <xt:vars title="Einladung" type="email" language="DE">
        <xt:param name="description">Einladungsemail</xt:param>
    </xt:vars>
</xt:locale>

I am loading the XML file with DOMDocument and would like to select the nodes with DOMXPath.

$template_content = file_get_contents($template_file);
$xml = new DOMDocument("1.0", "UTF8");
$xml->preserveWhiteSpace = false;
if ($xml->loadHTML($template_content)){
    $xpath = new DOMXPath($xml);
    $xpath->registerNamespace('xt', 'http://www.xelos.net/xt');
    $query = "//locale/vars[@language='{$language}']";
    $entries = $xpath->query($query);
    $xmlContent = '';
    foreach($entries as $entry) {
        $xmlContent = $entry->ownerDocument->saveXML($entry);
    }
    if ($xmlContent) {
        $template_content = $xmlContent;
        dump($template_content);
    }
}

I am not sure what the correct XPath selector is to grab the correct xt:vars node.