You have to buy all tickets and passes at machines. All of the machines have an English option. In the case of the Navigo Découverte, for instance, you purchase the pass, and the machine ejects a receipt. You take the receipt to a window where the clerk gives you the pass kit. Sometimes the clerk will put the pass together. Other times, if it's busy, you need to do it yourself.
It's dead simple. Bring a head shot of yourself that's a little smaller than a passport photo. You can even crop a passport photo to fit.
You stick your picture on the space with the adhesive and print your name in the space provided, fold it and insert it into the plastic case. Et voilà.
I've never seen murderously long lines at a Métro window.