Revert changes to language_link.js

Fixes #1491

Author: ihalaij1

exercise/static/exercise/language_link.js

@@ -1,31 +1,26 @@
-(function () {
-  const set_language_url = document.currentScript ?
-    document.currentScript.getAttribute('data-set-lang') :
-    document.querySelectorAll('script')[document.querySelectorAll('script').length - 1].getAttribute('data-set-lang');
+(function ($) {
+  const set_language_url = (document.currentScript ?
+    $(document.currentScript) :
+    $('script').last()) // Ugly solution for IE11
+    .attr('data-set-lang');
 
-  document.addEventListener('DOMContentLoaded', function () {
-    document.querySelectorAll('a.change-language').forEach(function (elem) {
-      const target_lang = elem.getAttribute('data-target-lang');
-      elem.addEventListener('click', function (event) {
-        event.preventDefault();
-        fetch(set_language_url, {
-          method: 'POST',
-          headers: {
-            'Content-Type': 'application/json'
-          },
-          body: JSON.stringify({ language: target_lang })
-        })
-        .then(function (response) {
-          if (response.ok) {
-            const href = elem.getAttribute('href');
+  $(function () {
+    $('a.change-language').each(function (i, elem) {
+      const $elem = $(elem);
+      const target_lang = $elem.attr('data-target-lang');
+      $elem.on('click', function () {
+        $.post(set_language_url, { language: target_lang })
+          .done(function () {
+            const href = $elem.attr('href');
+            // href="#" means "this page, but in another language"
             if (href === '#') {
               window.location.reload();
             } else {
               window.location.href = href;
             }
-          }
-        });
+          });
+        return false;
       });
     });
   });
-})();
+})(jQuery);