Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/language/language.admin.js

@@ -0,0 +1,40 @@
+/**
+ * @file
+ * Language admin behavior.
+ */
+
+(function ($, Drupal) {
+
+  "use strict";
+
+  /**
+   * Makes language negotiation inherit user interface negotiation.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.negotiationLanguage = {
+    attach: function () {
+      var $configForm = $('#language-negotiation-configure-form');
+      var inputSelector = 'input[name$="[configurable]"]';
+      // Given a customization checkbox derive the language type being changed.
+      function toggleTable(checkbox) {
+        var $checkbox = $(checkbox);
+        // Get the language detection type such as Interface text language
+        // detection or Content language detection.
+        $checkbox.closest('.table-language-group')
+          .find('table, .tabledrag-toggle-weight')
+          .toggle($checkbox.prop('checked'));
+      }
+
+      // Bind hide/show and rearrange customization checkboxes.
+      $configForm.once('negotiation-language-admin-bind').on('change', inputSelector, function (event) {
+        toggleTable(event.target);
+      });
+      // Initially, hide language detection types that are not customized.
+      $configForm.find(inputSelector + ':not(:checked)').each(function (index, element) {
+        toggleTable(element);
+      });
+    }
+  };
+
+})(jQuery, Drupal);