Do not try to set the locale in a stateless request

src/EventListener/LocaleListener.php

@@ -17,6 +17,12 @@ final readonly class LocaleListener
     public function onKernelRequest(RequestEvent $event): void
     {
         $request = $event->getRequest();
-        $this->localeSwitcher->setLocale($request->getSession()->get('_locale', 'en'));
+        if ($request->attributes->getBoolean('_stateless')) {
+            return;
+        }
+        $locale = $request->getSession()->get('_locale');
+        if ($locale !== null) {
+            $this->localeSwitcher->setLocale($locale);
+        }
     }
 }