datePickerProps={{ weekStartDay: 1 }} in DatePicker component has no effect

I use version 11.1.5 of the AtlasKit DatePicker. And despite the main language being English, in my app the date picker shows Sunday as the first weekday instead of Monday. So I added a datePickerProp:

    onChange={(date) => this.onDateChange(date)}
    value={isDisabled ? '' : date}
    datePickerProps={{ weekStartDay: 1 }}

But there is no change:

What’s missing? Or what is the best practice here, to also honour user settings.