private function loadDatabase(string $databaseFile): void
    {
        $isoNumber = $this->getISONumber();

        // add gettext domain
        \bindtextdomain(
            $isoNumber,
            $this->getLocalMessagesDirPath()
        );

        \bind_textdomain_codeset(
            $isoNumber,
            'UTF-8'
        );

        // load database from json file
        $json = \json_decode(
            file_get_contents($databaseFile),
            true
        );

        // build cluster index from database
        $this->clusterIndex = $json[$isoNumber];
    }