[5.x] Fix localized error messages on forms when previous URL is inco…

…rrect (#11219)

src/Http/Requests/FrontendFormRequest.php

@@ -120,8 +120,15 @@ public function validateResolved()
         // If this was submitted from a front-end form, we want to use the appropriate language
         // for the translation messages. If there's no previous url, it was likely submitted
         // directly in a headless format. In that case, we'll just use the default lang.
-        $site = ($previousUrl = session()->previousUrl()) ? Site::findByUrl($previousUrl) : null;
+        $site = ($previousUrl = $this->previousUrl()) ? Site::findByUrl($previousUrl) : null;
 
         return $this->withLocale($site?->lang(), fn () => parent::validateResolved());
     }
+
+    private function previousUrl()
+    {
+        return ($referrer = request()->header('referer'))
+            ? url()->to($referrer)
+            : session()->previousUrl();
+    }
 }