diff --git a/core/modules/language/src/LanguageNegotiationMethodInterface.php b/core/modules/language/src/LanguageNegotiationMethodInterface.php
index 0488516f2dd69a9b82f1febd92c2a3798811cd0f..2c1cf3ac8e791c080360032a7b107138f2a0560d 100644
--- a/core/modules/language/src/LanguageNegotiationMethodInterface.php
+++ b/core/modules/language/src/LanguageNegotiationMethodInterface.php
@@ -44,8 +44,15 @@ public function setCurrentUser(AccountInterface $current_user);
    *   (optional) The current request. Defaults to NULL if it has not been
    *   initialized yet.
    *
-   * @return string
-   *   A valid language code or FALSE if the negotiation was unsuccessful.
+   * @return string|null|false
+   *   A valid language code if the negotiation was successful and either NULL
+   *    or FALSE otherwise.
+   *
+   * @todo Determine whether string|false or string|null should be the
+   *   normalized result across all implementations and update the @return and
+   *   its comment accordingly.
+   *
+   * @see https://www.drupal.org/node/3329952
    */
   public function getLangcode(Request $request = NULL);