Inserted en-US slugs

src/olympia/pages/templates/pages/dev_faq.html

@@ -71,8 +71,8 @@
       {% endtrans %}
     </p>
     <p>
-      {% trans webext_url="https://developer.mozilla.org/Add-ons/WebExtensions",
-               portlegacy_url="https://developer.mozilla.org/Add-ons/WebExtensions/Porting_a_legacy_Firefox_add-on"%}
+      {% trans webext_url="https://developer.mozilla.org/en-US/Add-ons/WebExtensions",
+               portlegacy_url="https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_a_legacy_Firefox_add-on"%}
       <strong>Note:</strong> XUL is considered a legacy technology in Firefox, and add-ons using XUL will no longer load starting from 
       Firefox 57. Please use <a href="{{ webext_url }}">WebExtensions</a> APIs to develop new add-ons. If you have a legacy add-on, 
       consider migrating it to WebExtensions. For information on migrating a legacy add-on, please visit