I've just tried it here and could reproduce the issue in ExtJS 6.5.1 with this simple code:

Code:
Ext.application({
    name : 'Fiddle',

    launch : function() {
        Ext.create('Ext.form.field.Date', {
            renderTo: Ext.getBody()
        });
		
	Ext.create('Ext.picker.Date', {
            renderTo: Ext.getBody(),
            margin: '10px'
        });
    }
});
But in fiddle I can't choose the "es" (spanish) locale, can I? When the locale is spanish, then the fixed picker (date picker) days of week are L M M J V S D. From a DateField, expanding the picker gives week days named after D L M M J V S.

So I guess this is still broken.

Here's what I see if using the 'es' locale:
20791-esDatePickers.png