To localize the day names please override DateHelper.locale property. Change 'en-US' to 'es-ES'.
DateHelper : {
locale : 'es-ES',
When create a new locale please register it. See registerLocale. You can also extend one of existing locales. To apply new locale please use applyLocale
// ganttlocale is your locale definitions
LocaleManager.registerLocale('Es', { desc : 'Spanish', locale : ganttlocale });
And apply locale
LocaleManager.applyLocale('Es');
We will update our guide. Thank you for the feedback!