Use workaround with two fields, lets call them 'field1' and 'field2'.
In field1 you will select date and then hide it to show field2 with desired format, when you click on field2 to change date it will be hidden and focus set to field1.
1. In field1 use format 'dd mmmm yyyy',
as 'On Blur' action use this:
if(event.value){
event.target.display = display.hidden;
this.getField("field2").display = display.visible;}
as 'Validation' script use this:
var str = event.value;
var date = str.split(" ");
if(event.value)
this.getField("field2").value = date[0]+" de "+date[1]+" de "+date[2];
else
this.getField("field2").value = "";
2. in field2 as 'On Focus' action use this:
event.target.display = display.hidden;
this.getField("field1").display = display.visible;
this.getField("field1").setFocus();
Now put fields on top of each other so they are in same place.
Here is sample how it should work:
https://drive.google.com/uc?export=download&id=16ZUdWggM7hwNW4Cf1Xq0RjEoUEoWAOWn