sitemap: Add alternate hreflang links when a page is translated (#150, #67)

layouts/sitemap.xml

@@ -1,8 +1,12 @@
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
   {{ range (where .Data.Pages "Section" "!=" "home") }}
     {{ $perm := add $.Site.LanguagePrefix "/home/" }}
     {{ if ne .RelPermalink $perm }}
     <url>
+    {{ if .IsTranslated }}
+      <xhtml:link rel="alternate" href="{{ .Permalink }}" hreflang="{{ .Lang }}"/>{{ range .Translations }}
+      <xhtml:link rel="alternate" href="{{ .Permalink }}" hreflang="{{ .Lang }}"/>{{ end }}
+    {{ end }}
       <loc>{{ .Permalink }}</loc>{{ if not .Lastmod.IsZero }}
       <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ end }}{{ with .Sitemap.ChangeFreq }}
       <changefreq>{{ . }}</changefreq>{{ end }}{{ if ge .Sitemap.Priority 0.0 }}