[IMP] _extension: add languages link

To refer the pages in different languages for better indexing

_extensions/odoo/layout.html

@@ -25,6 +25,13 @@
   {{ super() }}
 {%- endblock -%}
 
+{%- block linktags -%}
+  {% for code, url in language_codes %}
+    <link rel="alternate" hreflang="{{ code }}" href="{{ url }}" />
+  {%- endfor %}
+  {{ super() }}
+{%- endblock -%}
+
 {%- block sidebar1 -%}{%- endblock -%}
 {%- block sidebar2 -%}{%- endblock -%}
 {%- block relbar1 -%}{%- endblock -%}

conf.py

@@ -404,6 +404,10 @@ def localize(app, pagename, templatename, context, doctree):
         for la in app.config.languages.split(',')
         if la != current_lang
     ]
+    context['language_codes'] = [
+        (la.split('_')[0] if la != 'en' else 'x-default', _build_url(app.config.canonical_root, (la != 'en' and la or ''), pagename))
+        for la in app.config.languages.split(',')
+    ]
 
 
 def _build_url(root, branch, pagename):