Backend translates the search results page

Now the search results page is presented by the backend in the language
controlled by the value of the `userlang` URL query parameter (or, if
the latter is missing, the value of the `Accept-Language:` HTTP header).

Note that the front-end doesn't yet take advantage of this
functionality.

src/search_renderer.cpp

@@ -192,7 +192,6 @@ kainjow::mustache::data buildPagination(
 
 std::string SearchRenderer::renderTemplate(const std::string& tmpl_str, const NameMapper& nameMapper, const Library* library)
 {
-  const std::string userlang("en");
   const std::string absPathPrefix = protocolPrefix;
   // Build the results list
   kainjow::mustache::data items{kainjow::mustache::data::type::list};

src/server/internalServer.cpp

@@ -967,6 +967,7 @@ std::unique_ptr<Response> InternalServer::handle_search_request(const RequestCon
   renderer.setProtocolPrefix(m_root + "/content/");
   renderer.setSearchProtocolPrefix(m_root + "/search");
   renderer.setPageLength(pageLength);
+  renderer.setUserLang(request.get_user_language());
   if (request.get_requested_format() == "xml") {
     return ContentResponse::build(
       renderer.getXml(*mp_nameMapper, mp_library.get()),