We operate in several countries. How can I make sure that in calculations GuiXT uses the right decimal separator, comma or point?

Please use the option "decimalseparator=userdefault" in Set. If you need to know explicitly whether point or comma is the user default, you can employ the following method:

Set V[value] 11 / 10  decimalseparator=userdefault
Set V[decimalseparator] "&V[value](2-2)"

Please observe that you need a valid RFC user in GuiXT profile for the "userdefault" feature.