Implement localisation for Google Sitemaps

GoogleSitemapController_sitemap.ss

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type='text/xsl' href='{$BaseHref}googlesitemaps/templates/xml-sitemap.xsl'?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
	<% loop $Items %>
		<url>
			<loc>$AbsoluteLink</loc><% if Localisations %><% loop Localisations %>
			<xhtml:link rel="alternate" hreflang="$PlaceHolderForLocale" href="$AbsoluteLink" /><% end_loop %><% end_if %>
			<% if $LastEdited %><lastmod>$LastEdited.Format(c)</lastmod><% end_if %>
			<% if $ChangeFrequency %><changefreq>$ChangeFrequency</changefreq><% end_if %>
			<% if $GooglePriority %><priority>$GooglePriority</priority><% end_if %>
		</url>
	<% end_loop %>
</urlset>

LocalisationDataExtension.php

<?php

class LocalisationDataExtension extends DataExtension {

	public function getLocalisations() {
		$locales = Translatable::get_existing_content_languages();
		$currentLocale = Translatable::get_current_locale();

		$output = new ArrayList();

		foreach($locales as $locale => $name) {
			if($page = $this->owner->getRelatedLocalisation($locale, $currentLocale)) {
				if($page && $page->exists()) {
					$page->PlaceHolderForLocale = $this->updateLocaleForGoogle($locale);
				
					$output->push($page);
				}	
			}

		}

		return $output;
	}
	
	public function updateLocaleForGoogle($locale) {
		return strtolower(str_replace("_", "-", $locale));
	}

}