Can not change firstDayOfWeek of calendar in qml

[tamolo]

In Calendar qml doc http://doc.qt.io/qt-5/qml-qtquick-controls-calendar.html#dayOfWeekFormat-prop i do not see any property to change the firstDayOfWeek. So i look into calendar.qml source code and i saw a property __locale

/*!
    The locale that this calendar should use to display itself.

    Affects how dates and day names are localized, as well as which
    day is considered the first in a week.

    To set an Australian locale, for example:

    \code
    locale: Qt.locale("en_AU")
    \endcode

    The default locale is \c Qt.locale().
*/
property var __locale: Qt.locale()

An object created by Qt.locale() will have a property firstDayOfWeek
http://doc.qt.io/qt-5/qml-qtqml-locale.html#firstDayOfWeek-prop

I want to change the first day of week of a calendar so i set

Calendar {
    id: cal
    Component.onCompleted: {
        cal.__locale.firstDayOfWeek = Locale.Tuesday;
   }
}

But the firstDayOfWeek do not change
Can anyone tell me what did i do wrong ?
Is there any other ways to change the firstDayOfWeek of calendar in qml ?