Label figure component as English

When pages are translated, there is no translation on the alt
text or the caption attached to an image, so they need to be
semantically labelled as being in English.

app/views/components/_figure.html.erb

@@ -3,7 +3,7 @@
   caption ||= ''
   credit ||= ''
 %>
-<figure class="app-c-figure">
+<figure class="app-c-figure" lang="en">
   <img class="app-c-figure__image" src="<%= src %>" alt="<%= alt %>">
     <figcaption class="app-c-figure__figcaption">
       <% unless caption.blank? %>