private function getPageMap(string $sourceLanguage, string $targetLanguage): PageMap
    {
        if (!array_key_exists($key = $sourceLanguage . '->' . $targetLanguage, $this->pageMaps)) {
            return $this->pageMaps[$key] = new PageMap(
                $sourceLanguage,
                $targetLanguage,
                $this->database,
                $this->logger
            );
        }

        return $this->pageMaps[$key];
    }