private function getLocalizedLabelsFromFile($extensionKey)
    {
        if ($extensionKey === '') {
            throw new \InvalidArgumentException('$extensionKey must not be empty.', 1331489618);
        }

        /** @var LocalizationFactory $languageFactory */
        $languageFactory = GeneralUtility::makeInstance(LocalizationFactory::class);
        $languageFile = 'EXT:' . $extensionKey . '/' . self::LANGUAGE_FILE_PATH;
        $localizedLabels = $languageFactory->getParsedData($languageFile, $this->languageKey, 'utf-8', 0);

        if ($this->alternativeLanguageKey !== '') {
            $alternativeLabels = $languageFactory->getParsedData($languageFile, $this->languageKey, 'utf-8', 0);
            $localizedLabels = array_merge(
                $alternativeLabels,
                is_array($localizedLabels) ? $localizedLabels : []
            );
        }

        return $localizedLabels;
    }