Update to Drupal 8.2.6. For more information, see https://www.drupal.org/project/drupal/releases/8.2.6

vendor/symfony/translation/LoggingTranslator.php

@@ -88,6 +88,20 @@ class LoggingTranslator implements TranslatorInterface, TranslatorBagInterface
         return $this->translator->getCatalogue($locale);
     }
 
+    /**
+     * Gets the fallback locales.
+     *
+     * @return array $locales The fallback locales
+     */
+    public function getFallbackLocales()
+    {
+        if ($this->translator instanceof Translator) {
+            return $this->translator->getFallbackLocales();
+        }
+
+        return array();
+    }
+
     /**
      * Passes through all unknown calls onto the translator object.
      */