Alter your WordPress page meta (like changing the html element) based on post template

functions.php

function prefix_template_meta() {

    if (get_page_template_slug() == 'page template slug') {

        // This is an arbitrary operation, in this case. You could do anything here.
        // Beware making assumptions about the availability of things, like the post ID, if you change the action hook.
        add_filter( 'language_attributes', 'prefix_xml_namespaces' );
    }
}

add_action('wp','prefix_template_meta');


function prefix_xml_namespaces($output) {

    return $output . ' xmlns="http://www.w3.org/1999/xhtml"';

}

May want to filter out the admin, etc.