public function setAcceptLanguage($accept_language)
    {
        $accept_language = strtolower($accept_language);

        if (!in_array($accept_language, [self::ACCEPT_LANGUAGE_EN, self::ACCEPT_LANGUAGE_DE])) {
            throw new \InvalidArgumentException('Parameter "accept_language" must be valid');
        }

        $this->accept_language = $accept_language;
        return $this;
    }