I think as long as you are courteous, you shouldn't have a problem buying the Carte Orange. It is always nice to adress:
Monsieur (Mr.)
Madame (Mrs)
Mademoiselle (Miss)
when you get to the window.
After you ask for the Carte Orange, say
S'il vous plait (Please)

You should also know in advance which zones you want to get. Better yet, write/print on a piece of paper:

Carte Orange
1 semaine
Zone 1-2 (or however many zones you want)

And say Merci afterwards.

If the ticket agent refuses to sell you, walk to the next Metro station and try again.

Good luck!