Ensure we always have an up to date list of language names

gui/index.html

@@ -1129,6 +1129,7 @@
   <script src="scripts/syncthing/core/services/localeService.js"></script>
 
   <script src="assets/lang/valid-langs.js"></script>
+  <script src="assets/lang/prettyprint.js"></script>
   <script src="scripts/syncthing/app.js"></script>
   <!-- / gui application code -->