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

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