As I recall (over 20 years ago) the formal address (as in a meeting) was always "brother or sister + surname" right after baptism, never before. A non-baptised would be called "Mr., Mrs. or Ms" -- but in informal situations people would generally call each other by their first names, baptised or unbaptised.
Another French feature is that you had to use the informal (or more intimate) you-address ("tu") instead of the more formal one ("vous") between baptised JWs, regardless of age or social relationship. Which was a bit strange to non-JW onlookers. I remember the case of a JW woman in her 40s, married to a non-JW man, both very high-class. They would use the formal "vous" when addressing to each other (it's unusual, but it happens), but I (in my early 20s back then) would use the informal "tu" when addressing her, in her husband's presence, which was weird.
In Portuguese congregations I sometimes heard people calling elderly women by combining the equivalents of "sister" and "lady" (irmâ dona X), because "sister" was deemed disrespectful.