diff --git a/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSession.php b/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSession.php
index f36d43e..7d10a88 100644
--- a/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSession.php
+++ b/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSession.php
@@ -109,14 +109,19 @@ public function processOutbound($path, &$options = [], Request $request = NULL,
           // Cached URLs that have been processed by this outbound path
           // processor must be:
           $bubbleable_metadata
-            // - invalidated when the language negotiation config changes, since
-            //   another query parameter may be used to determine the language.
+              // - invalidated when the language negotiation config changes, since
+              //   another query parameter may be used to determine the language.
             ->addCacheTags($this->config->get('language.negotiation')->getCacheTags())
-            // - varied by the configured query parameter.
+              // - varied by the configured query parameter.
             ->addCacheContexts(['url.query_args:' . $this->queryParam]);
         }
       }
     }
+    // To avoid page cache duplicates, remove the language query if the default
+    // language is set.
+    if ($this->currentUser->isAnonymous() && $cache && isset($options['query'][$this->queryParam]) && $options['query'][$this->queryParam] == $default) {
+      unset($options['query'][$this->queryParam]);
+    }
     return $path;
   }
 
@@ -145,11 +150,16 @@ public function getLanguageSwitchLinks(Request $request, $type, Url $url) {
         $links[$langcode]['query'][$param] = $langcode;
       }
       else {
-        $links[$langcode]['attributes']['class'][] = 'session-active';
+        if ($langcode == $language_query) {
+          $links[$langcode]['attributes']['class'][] = 'session-active';
+        }
+        // Always set the language query parameter. It may be removed in
+        // LanguageNegotiationSession::processOutbound().
+        $links[$langcode]['query'][$param] = $langcode;
       }
-    }
 
-    return $links;
+      return $links;
+    }
   }
 
 }