I had a bad elbow break in week 5 of a 10 week trip which I certainly didn't want to end and had surgery and was in the hospital 5 days in southern France (in the US it would be outpatient surgery). My biggest issue right away since I hadn't planned for it was clothes. It was warm in the south so my tank tops were enough but it was going into fall as we headed north and nothing I had that was warm would fit over my arm. I managed to fine a cheap clothing store in Auxerre and buy a couple of things. So of course you want to pack for a cast -- but you have the advantage of being able to plan that ahead.
Casts are dangerous on planes due to swelling possibilities -- of course arm less so than legs or ankles, but your doctor may want you in a strap on splint rather than a cast so you don't have a mid air crisis. By the time I flew back I was in a splint. This is something to discuss with your doctor; at 30K feet and an arm being strangled in a cast you are in real trouble. I broke a wrist about 35 years ago and had a strap on splint type cast the whole time; I would think that would be much safer on a plane.
You will want to plan airline seating so you don't have a stranger seated next to your bad arm.
Expect airline security in France to totally lack sympathy. They made me unwrap the arm which was in a splint and were totally nasty as I struggled with my baggage and then my unwrapped vulnerable arm and the splint etc. They were mad because I was holding up the line through security trying to manage the mess they had made of it. Because I had the arm to manage and my stuff it was really awkward getting my stuff together and through the process and getting the arm secured again.
With just the forearm in a cast and time to plan clothes, having bags to cover the arm during showers etc etc, it should not affect your trip much. And along the way you will meet people who will be extra helpful (not at CDG but everywhere else). The hotel people in Auxerre could not have been nicer about rushing around and getting my bags to the room and the cab driver in Paris came into our building to manage my bags
I'm sure you will have a great trip and this will just be a petty annoyance.