diff --git a/core/lib/Drupal/Core/Session/AccountInterface.php b/core/lib/Drupal/Core/Session/AccountInterface.php
index 084943b..ede2854 100644
--- a/core/lib/Drupal/Core/Session/AccountInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountInterface.php
@@ -90,28 +90,34 @@ public function isAnonymous();
   /**
    * Returns the preferred language code of the account.
    *
-   * @param string $default
-   *   (optional) Default language code to return if the account
-   *   has no valid language, defaults to the site default language.
+   * @param bool $fallback_to_default
+   *   (optional) Whether the return value will fall back to the site default
+   *   language if the user has no language preference.
    *
    * @return string
-   *   The language code that is preferred by the account.
+   *   The language code that is preferred by the account. If the preferred
+   *   language is not set or is a language not configured anymore on the site,
+   *   the site default is returned or an empty string is returned (if
+   *   $fallback_to_default is FALSE).
    */
-  public function getPreferredLangcode($default = NULL);
+  public function getPreferredLangcode($fallback_to_default = TRUE);
 
   /**
    * Returns the preferred administrative language code of the account.
    *
    * Defines which language is used on administrative pages.
    *
-   * @param string $default
-   *   (optional) Default language code to return if the account
-   *   has no valid language, defaults to the site default language.
+   * @param bool $fallback_to_default
+   *   (optional) Whether the return value will fall back to the site default
+   *   language if the user has no administration language preference.
    *
    * @return string
-   *   The language code that is preferred by the account.
+   *   The language code that is preferred by the account for administration
+   *   pages. If the preferred language is not set or is a language not
+   *   configured anymore on the site, the site default is returned or an empty
+   *   string is returned (if $fallback_to_default is FALSE).
    */
-  public function getPreferredAdminLangcode($default = NULL);
+  public function getPreferredAdminLangcode($fallback_to_default = TRUE);
 
   /**
    * Returns the username of this account.
diff --git a/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
index 56a42fa..89e9146 100644
--- a/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -147,15 +147,15 @@ public function isAnonymous() {
   /**
    * {@inheritdoc}
    */
-  public function getPreferredLangcode($default = NULL) {
-    return $this->getAccount()->getPreferredLangcode($default);
+  public function getPreferredLangcode($fallback_to_default = TRUE) {
+    return $this->getAccount()->getPreferredLangcode($fallback_to_default);
   }
 
   /**
    * {@inheritdoc}
    */
-  public function getPreferredAdminLangcode($default = NULL) {
-    return $this->getAccount()->getPreferredAdminLangcode($default);
+  public function getPreferredAdminLangcode($fallback_to_default = TRUE) {
+    return $this->getAccount()->getPreferredAdminLangcode($fallback_to_default);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Session/UserSession.php b/core/lib/Drupal/Core/Session/UserSession.php
index 4a76326..7364da9 100644
--- a/core/lib/Drupal/Core/Session/UserSession.php
+++ b/core/lib/Drupal/Core/Session/UserSession.php
@@ -182,26 +182,26 @@ public function isAnonymous() {
   /**
    * {@inheritdoc}
    */
-  function getPreferredLangcode($default = NULL) {
+  function getPreferredLangcode($fallback_to_default = TRUE) {
     $language_list = language_list();
     if (!empty($this->preferred_langcode) && isset($language_list[$this->preferred_langcode])) {
       return $language_list[$this->preferred_langcode]->id;
     }
     else {
-      return $default ? $default : language_default()->id;
+      return $fallback_to_default ? language_default()->id : '';
     }
   }
 
   /**
    * {@inheritdoc}
    */
-  function getPreferredAdminLangcode($default = NULL) {
+  function getPreferredAdminLangcode($fallback_to_default = TRUE) {
     $language_list = language_list();
     if (!empty($this->preferred_admin_langcode) && isset($language_list[$this->preferred_admin_langcode])) {
       return $language_list[$this->preferred_admin_langcode]->id;
     }
     else {
-      return !is_null($default) ? $default : language_default()->id;
+      return $fallback_to_default ? language_default()->id : '';
     }
   }
 
diff --git a/core/modules/user/src/AccountForm.php b/core/modules/user/src/AccountForm.php
index 73dc470..c9451a6 100644
--- a/core/modules/user/src/AccountForm.php
+++ b/core/modules/user/src/AccountForm.php
@@ -248,7 +248,7 @@ public function form(array $form, FormStateInterface $form_state) {
 
     $user_preferred_langcode = $register ? $language_interface->id : $account->getPreferredLangcode();
 
-    $user_preferred_admin_langcode = $register ? $language_interface->id : $account->getPreferredAdminLangcode('');
+    $user_preferred_admin_langcode = $register ? $language_interface->id : $account->getPreferredAdminLangcode(FALSE);
 
     // Is the user preferred language added?
     $user_language_added = FALSE;
diff --git a/core/modules/user/src/Entity/User.php b/core/modules/user/src/Entity/User.php
index 6e2d44c..eeb8d55 100644
--- a/core/modules/user/src/Entity/User.php
+++ b/core/modules/user/src/Entity/User.php
@@ -366,28 +366,28 @@ public function getTimeZone() {
   /**
    * {@inheritdoc}
    */
-  function getPreferredLangcode($default = NULL) {
+  function getPreferredLangcode($fallback_to_default = TRUE) {
     $language_list = language_list();
     $preferred_langcode = $this->get('preferred_langcode')->value;
     if (!empty($preferred_langcode) && isset($language_list[$preferred_langcode])) {
       return $language_list[$preferred_langcode]->id;
     }
     else {
-      return $default ? $default : language_default()->id;
+      return $fallback_to_default ? language_default()->id : '';
     }
   }
 
   /**
    * {@inheritdoc}
    */
-  function getPreferredAdminLangcode($default = NULL) {
+  function getPreferredAdminLangcode($fallback_to_default = TRUE) {
     $language_list = language_list();
     $preferred_langcode = $this->get('preferred_admin_langcode')->value;
     if (!empty($preferred_langcode) && isset($language_list[$preferred_langcode])) {
       return $language_list[$preferred_langcode]->id;
     }
     else {
-      return !is_null($default) ? $default : language_default()->id;
+      return $fallback_to_default ? language_default()->id : '';
     }
   }
 
diff --git a/core/modules/user/src/Plugin/LanguageNegotiation/LanguageNegotiationUserAdmin.php b/core/modules/user/src/Plugin/LanguageNegotiation/LanguageNegotiationUserAdmin.php
index 3ce8184..4697dfa 100644
--- a/core/modules/user/src/Plugin/LanguageNegotiation/LanguageNegotiationUserAdmin.php
+++ b/core/modules/user/src/Plugin/LanguageNegotiation/LanguageNegotiationUserAdmin.php
@@ -93,7 +93,7 @@ public function getLangcode(Request $request = NULL) {
     $langcode = NULL;
 
     // User preference (only for administrators).
-    if ($this->currentUser->hasPermission('access administration pages') && ($preferred_admin_langcode = $this->currentUser->getPreferredAdminLangcode('')) && $this->isAdminPath($request)) {
+    if ($this->currentUser->hasPermission('access administration pages') && ($preferred_admin_langcode = $this->currentUser->getPreferredAdminLangcode(FALSE)) && $this->isAdminPath($request)) {
       $langcode = $preferred_admin_langcode;
     }