diff --git a/core/includes/update.inc b/core/includes/update.inc
index 9a4606e..4ae0302 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -747,7 +747,7 @@ function update_language_list($flags = LanguageInterface::STATE_CONFIGURABLE) {
     $default = isset($default) ? $default : \Drupal::languageManager()->getDefaultLanguage();
     // Rename the default language.
     $default->name = t("Site's default language (@lang_name)", array('@lang_name' => $default->name));
-    $filtered_languages['site_default'] = $default;
+    $filtered_languages[LanguageInterface::LANGCODE_SITE_DEFAULT] = $default;
   }
 
   foreach ($languages as $langcode => $language) {
diff --git a/core/lib/Drupal/Core/Language/LanguageInterface.php b/core/lib/Drupal/Core/Language/LanguageInterface.php
index f2e58f3..bb83e28 100644
--- a/core/lib/Drupal/Core/Language/LanguageInterface.php
+++ b/core/lib/Drupal/Core/Language/LanguageInterface.php
@@ -52,6 +52,11 @@
   const LANGCODE_DEFAULT = 'x-default';
 
   /**
+   * Language code referring to site's default language.
+   */
+  const LANGCODE_SITE_DEFAULT = 'site_default';
+
+  /**
    * The language state when referring to configurable languages.
    */
   const STATE_CONFIGURABLE = 1;
diff --git a/core/lib/Drupal/Core/Language/LanguageManager.php b/core/lib/Drupal/Core/Language/LanguageManager.php
index 35a3fad..2d15a95 100644
--- a/core/lib/Drupal/Core/Language/LanguageManager.php
+++ b/core/lib/Drupal/Core/Language/LanguageManager.php
@@ -150,7 +150,7 @@ public function getLanguages($flags = LanguageInterface::STATE_CONFIGURABLE) {
       // if we're acting on a global object, so clone the object first.
       $default = clone $default;
       $default->name = $this->t("Site's default language (@lang_name)", array('@lang_name' => $default->name));
-      $filtered_languages['site_default'] = $default;
+      $filtered_languages[LanguageInterface::LANGCODE_SITE_DEFAULT] = $default;
     }
 
     foreach ($this->languages as $id => $language) {
diff --git a/core/modules/language/language.module b/core/modules/language/language.module
index d60d42e..cd1d45bc 100644
--- a/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -189,7 +189,7 @@ function language_element_info() {
  */
 function language_configuration_element_default_options() {
   $language_options = array(
-    'site_default' => t("Site's default language (!language)", array('!language' => \Drupal::languageManager()->getDefaultLanguage()->name)),
+    LanguageInterface::LANGCODE_SITE_DEFAULT => t("Site's default language (!language)", array('!language' => \Drupal::languageManager()->getDefaultLanguage()->name)),
     'current_interface' => t('Current interface language'),
     'authors_default' => t("Author's preferred language"),
   );
@@ -299,7 +299,7 @@ function language_get_default_configuration($entity_type, $bundle) {
   if (is_null($configuration)) {
     $configuration = array();
   }
-  $configuration += array('langcode' => 'site_default', 'language_show' => FALSE);
+  $configuration += array('langcode' => LanguageInterface::LANGCODE_SITE_DEFAULT, 'language_show' => FALSE);
   return $configuration;
 }
 
@@ -361,13 +361,13 @@ function language_get_default_langcode($entity_type, $bundle) {
   $configuration = language_get_default_configuration($entity_type, $bundle);
 
   if (!isset($configuration['langcode'])) {
-    $configuration['langcode'] = 'site_default';
+    $configuration['langcode'] = LanguageInterface::LANGCODE_SITE_DEFAULT;
   }
 
   $default_value = NULL;
   $language_interface = \Drupal::languageManager()->getCurrentLanguage();
   switch ($configuration['langcode']) {
-    case 'site_default':
+    case LanguageInterface::LANGCODE_SITE_DEFAULT:
       $default_value = \Drupal::languageManager()->getDefaultLanguage()->id;
       break;
 
diff --git a/core/modules/language/src/Form/ContentLanguageSettingsForm.php b/core/modules/language/src/Form/ContentLanguageSettingsForm.php
index 34ce092..6361879 100644
--- a/core/modules/language/src/Form/ContentLanguageSettingsForm.php
+++ b/core/modules/language/src/Form/ContentLanguageSettingsForm.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -76,7 +77,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       // Check whether we have any custom setting.
       foreach ($bundles[$entity_type_id] as $bundle => $bundle_info) {
         $conf = language_get_default_configuration($entity_type_id, $bundle);
-        if (!empty($conf['language_show']) || $conf['langcode'] != 'site_default') {
+        if (!empty($conf['language_show']) || $conf['langcode'] != LanguageInterface::LANGCODE_SITE_DEFAULT) {
           $default[$entity_type_id] = $entity_type_id;
         }
         $language_configuration[$entity_type_id][$bundle] = $conf;
diff --git a/core/modules/language/src/Tests/LanguageConfigurationElementTest.php b/core/modules/language/src/Tests/LanguageConfigurationElementTest.php
index 6815955..9e0d285 100644
--- a/core/modules/language/src/Tests/LanguageConfigurationElementTest.php
+++ b/core/modules/language/src/Tests/LanguageConfigurationElementTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\language\Tests;
 
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\simpletest\WebTestBase;
 
@@ -86,7 +87,7 @@ public function testDefaultLangcode() {
     $this->assertTrue($configurable_language->isDefault(), 'The en language entity is flagged as the default language.');
 
     \Drupal::config('system.site')->set('langcode', 'cc')->save();
-    language_save_default_configuration('custom_type', 'custom_bundle', array('langcode' => 'site_default', 'language_show' => TRUE));
+    language_save_default_configuration('custom_type', 'custom_bundle', array('langcode' => LanguageInterface::LANGCODE_SITE_DEFAULT, 'language_show' => TRUE));
     $langcode = language_get_default_langcode('custom_type', 'custom_bundle');
     $this->assertEqual($langcode, 'cc');
 
diff --git a/core/modules/node/src/Tests/NodeTypeInitialLanguageTest.php b/core/modules/node/src/Tests/NodeTypeInitialLanguageTest.php
index 81bf814..971d091 100644
--- a/core/modules/node/src/Tests/NodeTypeInitialLanguageTest.php
+++ b/core/modules/node/src/Tests/NodeTypeInitialLanguageTest.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\node\Tests;
 
+use Drupal\Core\Language\LanguageInterface;
+
 /**
  * Tests node type initial language settings.
  *
@@ -36,7 +38,7 @@ protected function setUp() {
    */
   function testNodeTypeInitialLanguageDefaults() {
     $this->drupalGet('admin/structure/types/manage/article');
-    $this->assertOptionSelected('edit-language-configuration-langcode', 'site_default', 'The default initial language is the site default.');
+    $this->assertOptionSelected('edit-language-configuration-langcode', LanguageInterface::LANGCODE_SITE_DEFAULT, 'The default initial language is the site default.');
     $this->assertNoFieldChecked('edit-language-configuration-language-show', 'Language selector is hidden by default.');
 
     // Tests if the language field cannot be rearranged on the manage fields tab.
diff --git a/core/modules/node/src/Tests/Views/NodeLanguageTest.php b/core/modules/node/src/Tests/Views/NodeLanguageTest.php
index 290d9d8..0947a1e 100644
--- a/core/modules/node/src/Tests/Views/NodeLanguageTest.php
+++ b/core/modules/node/src/Tests/Views/NodeLanguageTest.php
@@ -9,6 +9,7 @@
 
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\views\Plugin\views\PluginBase;
 
 /**
  * Tests node language fields, filters, and sorting.
@@ -180,7 +181,7 @@ public function testLanguages() {
     // filter is set to the site default language instead. This should just
     // show the English nodes, no matter what the content language is.
     $config = \Drupal::config('views.view.frontpage');
-    $config->set('display.default.display_options.filters.langcode.value', array('***LANGUAGE_site_default***' => '***LANGUAGE_site_default***'));
+    $config->set('display.default.display_options.filters.langcode.value', array(PluginBase::VIEWS_QUERY_LANGUAGE_SITE_DEFAULT => PluginBase::VIEWS_QUERY_LANGUAGE_SITE_DEFAULT));
     $config->save();
     foreach ($this->node_titles as $langcode => $titles) {
       $this->drupalGet(($langcode == 'en' ? '' : "$langcode/") . 'node');
diff --git a/core/modules/taxonomy/src/Tests/TermLanguageTest.php b/core/modules/taxonomy/src/Tests/TermLanguageTest.php
index cb64143..4264c0b 100644
--- a/core/modules/taxonomy/src/Tests/TermLanguageTest.php
+++ b/core/modules/taxonomy/src/Tests/TermLanguageTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\taxonomy\Tests;
 
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\language\Entity\ConfigurableLanguage;
 
 /**
@@ -97,7 +98,7 @@ function testDefaultTermLanguage() {
     // language is still correctly selected.
     \Drupal::config('system.site')->set('langcode', 'cc')->save();
     $edit = array(
-      'default_language[langcode]' => 'site_default',
+      'default_language[langcode]' => LanguageInterface::LANGCODE_SITE_DEFAULT,
       'default_language[language_show]' => TRUE,
     );
     $this->drupalPostForm('admin/structure/taxonomy/manage/' . $this->vocabulary->id(), $edit, t('Save'));
diff --git a/core/modules/views/src/Plugin/views/PluginBase.php b/core/modules/views/src/Plugin/views/PluginBase.php
index a92027d..7363d28 100644
--- a/core/modules/views/src/Plugin/views/PluginBase.php
+++ b/core/modules/views/src/Plugin/views/PluginBase.php
@@ -52,6 +52,13 @@
   const INCLUDE_NEGOTIATED = 16;
 
   /**
+   * Query string to indicate the site default language.
+   *
+   * @see \Drupal\Core\Language\LanguageInterface::LANGCODE_DEFAULT
+   */
+  const VIEWS_QUERY_LANGUAGE_SITE_DEFAULT = '***LANGUAGE_site_default***';
+
+  /**
    * Options for this plugin will be held here.
    *
    * @var array
@@ -402,13 +409,13 @@ protected function listLanguages($flags = LanguageInterface::STATE_ALL) {
     $list = array();
 
     // The Language Manager class takes care of the STATE_SITE_DEFAULT case.
-    // It comes in with ID set to 'site_default'. Since this is not a real
-    // language, surround it by '***LANGUAGE_...***', like the negotiated
-    // languages below.
+    // It comes in with ID set to LanguageInterface::LANGCODE_SITE_DEFAULT.
+    // Since this is not a real language, surround it by '***LANGUAGE_...***',
+    // like the negotiated languages below.
     $languages = $manager->getLanguages($flags);
     foreach ($languages as $id => $language) {
-      if ($id == 'site_default') {
-        $id = '***LANGUAGE_' . $id . '***';
+      if ($id == LanguageInterface::LANGCODE_SITE_DEFAULT) {
+        $id = PluginBase::VIEWS_QUERY_LANGUAGE_SITE_DEFAULT;
       }
       $list[$id] = t($language->name);
     }
@@ -448,7 +455,7 @@ public static function queryLanguageSubstitutions() {
 
     // Handle default language.
     $default = $manager->getDefaultLanguage()->id;
-    $changes['***LANGUAGE_site_default***'] = $default;
+    $changes[PluginBase::VIEWS_QUERY_LANGUAGE_SITE_DEFAULT] = $default;
 
     // Handle negotiated languages.
     $types = $manager->getDefinedLanguageTypesInfo();
diff --git a/core/modules/views/views.api.php b/core/modules/views/views.api.php
index 5534df6..3f73e63 100644
--- a/core/modules/views/views.api.php
+++ b/core/modules/views/views.api.php
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Language\LanguageInterface;
+use Drupal\views\Plugin\views\PluginBase;
 
 /**
  * @defgroup views_overview Views overview
@@ -579,7 +580,7 @@ function hook_views_query_substitutions(ViewExecutable $view) {
     '***CURRENT_VERSION***' => \Drupal::VERSION,
     '***CURRENT_TIME***' => REQUEST_TIME,
     '***LANGUAGE_language_content***' => \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->id,
-    '***LANGUAGE_site_default***' => \Drupal::languageManager()->getDefaultLanguage()->id,
+    PluginBase::VIEWS_QUERY_LANGUAGE_SITE_DEFAULT => \Drupal::languageManager()->getDefaultLanguage()->id,
   );
 }