public function setFallback($key) {
        $key = $this->canonicalize($key);

        if (!isset($this->_locales[$key])) {
            throw new MissingLocaleException(sprintf('Locale %s has not been setup', $key));
        }

        $this->_fallback = $this->_locales[$key];

        Config::set('titon.locale.fallback', $key);

        return $this;
    }