diff -u b/core/modules/language/language.module b/core/modules/language/language.module
--- b/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -431,15 +431,8 @@
   $language_entity->direction = isset($language->direction) ? $language->direction : '0';
   $language_entity->locked = !empty($language->locked);
   $language_entity->weight = isset($language->weight) ? $language->weight : 0;
+  $language_entity->setDefault(!empty($language->default));
   $language_entity->save();
-
-  if (!empty($language->default)) {
-    // Update the config. Saving the configuration fires and event that causes
-    // the container to be rebuilt.
-    \Drupal::config('system.site')->set('langcode', $language->id)->save();
-    \Drupal::service('language.default')->set($language);
-  }
-
   $t_args = array('%language' => $language->name, '%langcode' => $language->id);
   if ($language->is_new) {
     watchdog('language', 'The %language (%langcode) language has been created.', $t_args);
diff -u b/core/modules/language/lib/Drupal/language/Entity/Language.php b/core/modules/language/lib/Drupal/language/Entity/Language.php
--- b/core/modules/language/lib/Drupal/language/Entity/Language.php
+++ b/core/modules/language/lib/Drupal/language/Entity/Language.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\language\Entity;
 
+use Drupal\Core\Language\Language as LanguageObject;
 use Drupal\Core\Config\Entity\ConfigEntityBase;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\language\ConfigurableLanguageManager;
@@ -80,11 +81,32 @@
   public $locked = FALSE;
 
   /**
+   * Default site language.
+   *
+   * This property is not saved to the language entity since there can be only
+   * one default language. It is saved to system.site:langcode and set on the
+   * container using the language.default service in
+   * \Drupal\language\Entity\Language::postSave().
+   *
+   * @var bool
+   */
+  protected $default = FALSE;
+
+  /**
    * Used to detect when the site becomes multilingual.
    *
    * @var bool
    */
-  protected $preSaveMultilingual;
+  protected $multilingual;
+
+  /**
+   * Sets the default flag on the language entity.
+   *
+   * @param bool $default
+   */
+  public function setDefault($default) {
+    $this->default = $default;
+  }
 
   /**
    * {@inheritdoc}
@@ -107,6 +129,13 @@
   public function postSave(EntityStorageInterface $storage, $update = TRUE) {
     parent::postSave($storage, $update);
 
+    if (!empty($this->default)) {
+      // Update the config. Saving the configuration fires and event that causes
+      // the container to be rebuilt.
+      \Drupal::config('system.site')->set('langcode', $this->id())->save();
+      \Drupal::service('language.default')->set($this->toLanguageObject());
+    }
+
     $language_manager = \Drupal::languageManager();
     $language_manager->reset();
     if ($language_manager instanceof ConfigurableLanguageManagerInterface) {
@@ -118,14 +147,31 @@
     // flushed.
     language_negotiation_url_prefixes_update();
 
-    // If after adding this language the site will become multilingual, we need
-    // to rebuild language services.
-    if (!$this->preSaveMultilingual && !$update) {
+    // If after adding this language the site will become multilingual, we need to
+    // rebuild language services.
+    if (!$this->multilingual && !$update) {
       ConfigurableLanguageManager::rebuildServices();
     }
   }
 
   /**
+   * Converts the Language entity to a Language value object.
+   *
+   * @return \Drupal\Core\Language\Language
+   *   The language configuration entity expressed as a Language value object.
+   */
+  protected function toLanguageObject() {
+    return new LanguageObject(array(
+      'id' => $this->id(),
+      'name' => $this->label(),
+      'direction' => $this->direction,
+      'weight' => $this->weight,
+      'locked' => $this->locked,
+      'default' => $this->default,
+    ));
+  }
+
+  /**
    * {@inheritdoc}
    *
    * @throws \RuntimeException