Fix broken doc searching in built-in template

Analogue to changes made in the Read The Docs theme in
readthedocs/sphinx_rtd_theme#1021

doc/templates/bootstrap/search.html

@@ -11,6 +11,7 @@
 {% set title = _('Search') %}
 {% block extrahead %}
   <script type="text/javascript" src="{{ pathto('_static/searchtools.js', 1) }} "></script>
+  <script type="text/javascript" src="{{ pathto('_static/language_data.js', 1) }}"></script>
   <script type="text/javascript" src="{{ pathto('searchindex.js', 1) }} "></script>
   {# this is used when loading the search index using $.ajax fails,
      such as on Chrome for documents on localhost #}