I want to format a variable in order to display it in the European form "246.824,717" or alternatively in the Anglo-American form "246,824.717"
I tried (for the European form):
Set V[x] 246824717 Decimals=3 DecimalSeparator="," GroupSeparator="."
But the result is still "246824717" so what should I do?

GuiXT observes the additional formatting options only in calculations, not in "Set" alone. So you have to add 0 to the variable:
Set V[x] 246824717 + 0 Decimals=3 DecimalSeparator="," GroupSeparator="."