If you omit these niceties, you will be perceived as arrogant and brusque, and will be treated accordingly. If you take the time to observe these cultural practices, the French people you meet will be amazed and delighted, being used to Americans who don't know any better. You will find them truly friendly, even if your accent is less than perfect. They also love tandems as much as anybody, although they are less rare there than here. They are much richer in cycling skill.
Motorists have different attitudes and expectations than they do here.
I felt that the French motorists gave us a reasonable amount of room and yet were across the middle line for a minimal amount of time. At any event, I enjoyed being passed by cars in France. They made me think they knew what they were doing. France is paradise for the cycletourist. I've been only briefly to France, but do speak a little French.