add lang link refs

build/templates/index.template

@@ -58,7 +58,9 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 <title>{{title}}</title>
 
-<link rel="alternate" type="application/atom+xml" title="threejs fundamentals" href="https://threejsfundamentals.org/atom.xml" />
+{{{include "build/templates/langs-meta.template"}}}
+
+<link rel="alternate" type="application/atom+xml" title="threejs fundamentals" href="https://threejsfundamentals.org/atom.xml">
 <script type="application/ld+json">
 {
   "@context":"https://schema.org",

build/templates/langs-meta.template

@@ -0,0 +1,5 @@
+{{#each langs}}
+  <link rel="alternate" hreflang="{{lang}}" href="https://webglfundamentals.org{{url}}">
+{{/each}}
+
+

build/templates/lesson.template

@@ -56,6 +56,8 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 <meta name="twitter:description" content="{{description}}">
 <meta name="twitter:image:src" content="{{screenshot}}">
 
+{{{include "build/templates/langs-meta.template"}}}
+
 <script type="application/ld+json">
 {
   "@context":"https://schema.org",