private function setContaoLanguageDirectory(InputInterface $input, OutputInterface $output)
    {
        $this->ctolang = $input->getOption('contao');

        if ($this->ctolang === null) {
            $this->ctolang = $this->getTransifexConfigValue('/languages_cto');

            if ($this->ctolang === null) {
                throw new \RuntimeException('Error: unable to determine contao language root folder.');
            }
            $this->writelnVerbose(
                $output,
                sprintf('<info>automatically using Contao language folder: %s</info>', $this->ctolang)
            );
        }
    }