Issue #2940121 by apaderno, andypost, idebr, Kingdutch, kala4ek, mfb, alexpott, Berdir: ConfigurableLanguageManager::getLanguageSwitchLinks() doesn't respect what LanguageManagerInterface::getLanguageSwitchLinks() says to return

core/lib/Drupal/Core/Language/LanguageManager.php

@@ -215,7 +215,7 @@ class LanguageManager implements LanguageManagerInterface {
    * {@inheritdoc}
    */
   public function getLanguageSwitchLinks($type, Url $url) {
-    return [];
+    return NULL;
   }
 
   /**

core/lib/Drupal/Core/Language/LanguageManagerInterface.php

@@ -171,8 +171,11 @@ interface LanguageManagerInterface {
    * @param \Drupal\Core\Url $url
    *   The URL the switch links will be relative to.
    *
-   * @return array
-   *   A keyed array of links ready to be themed.
+   * @return object|null
+   *   An object with the following keys:
+   *   - links: An array of links indexed by the language ID
+   *   - method_id: The language negotiation method ID
+   *   or NULL if there are no language switch links.
    */
   public function getLanguageSwitchLinks($type, Url $url);
 

core/modules/language/src/ConfigurableLanguageManager.php

@@ -403,8 +403,6 @@ class ConfigurableLanguageManager extends LanguageManager implements Configurabl
    * {@inheritdoc}
    */
   public function getLanguageSwitchLinks($type, Url $url) {
-    $links = FALSE;
-
     if ($this->negotiator) {
       foreach ($this->negotiator->getNegotiationMethods($type) as $method_id => $method) {
         $reflector = new \ReflectionClass($method['class']);
@@ -422,7 +420,7 @@ class ConfigurableLanguageManager extends LanguageManager implements Configurabl
       }
     }
 
-    return $links;
+    return $links ?? NULL;
   }
 
   /**