Use metadata.language for the lang attribute

It's already `en` by default.

_includes/layouts/base.njk

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="en">
+<html lang="{{ metadata.language }}">
   <head>
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">