We are relatively formal, where I live.Just yesterday, I happened to meet my son's mother-in-law, and her sister, at the grocery store. I hadn't met this sister before, so son's MIL introduced me as "Mme.A."
Sometimes my son's MIL and I first-name each other, but more usually, I'd call her Mme.
Jacqueline (her first name), and she calls me Mme.Jean.
(should add, she is the nicest of women, and fond of my son, as he is of her.)