Make change frequency options translatable

/**
 * Gets xmlsitemap frequency options.
 *
 * @return array
 *   Frequency options.
 *
 * @todo Document this function.
 * @todo Make these translatable
 */
function xmlsitemap_get_changefreq_options() {
  return array(
    XMLSITEMAP_FREQUENCY_ALWAYS => 'always',
    XMLSITEMAP_FREQUENCY_HOURLY => 'hourly',
    XMLSITEMAP_FREQUENCY_DAILY => 'daily',
    XMLSITEMAP_FREQUENCY_WEEKLY => 'weekly',
    XMLSITEMAP_FREQUENCY_MONTHLY => 'monthly',
    XMLSITEMAP_FREQUENCY_YEARLY => 'yearly',
  );
}