Add 'en' language attribute to the Brexit callout

This section is not currently translated, though it can and does appear on
[some non-English pages](https://www.gov.uk/government/publications/trade-with-the-uk-from-january-2021-prepare-your-eu-business/handel-mit-dem-vereinigten-konigreich-ab-1-januar-2021-fur-unternehmen-mit-sitz-in-der-eu).

https://trello.com/c/uaDK0l8U/445-add-language-attribute-to-autogenerated-eu-transition-banner

Author: sihugh

app/views/components/_header-notice.html.erb

@@ -6,7 +6,7 @@
   featured_link ||= false
 %>
 <% if title && description %>
-  <section class="app-c-header-notice" aria-label="notice" role="region">
+  <section class="app-c-header-notice" aria-label="notice" role="region" lang="en">
     <div class="app-c-header-notice__header">
       <h2 class="app-c-header-notice__title"><%= title %></h2>
     </div>