Allow overriding HTML lang on a per-page basis (#4862)

The W3C [recommends](https://www.w3.org/International/questions/qa-html-language-declarations) to specify language using identifiers as per [RFC 5646](https://tools.ietf.org/html/rfc5646) which uses dashes.

_layouts/archive-taxonomy.html

@@ -20,7 +20,7 @@ author_profile: false
 
   <div class="archive">
     {% unless page.header.overlay_color or page.header.overlay_image %}
-      <h1 id="page-title" class="page__title">{{ page.title }}</h1>
+      <h1 id="page-title" class="page__title"{% if page.locale %} lang="{{ page.locale }}"{% endif %}>{{ page.title }}</h1>
     {% endunless %}
     {% for post in page.posts %}
       {% include archive-single.html %}