The Civil Registries are maintained by the clergy under the old regime (the acts of Baptism Marriage and Sepulture (BMS)), then after the law of September 20, 1792 it's a Civil Registrar who must keep up to date the informations on the births, marriages and deaths.
These registers have been digitized and are accessible from the Internet.

Drafting of deeds

The language used

They are written in Latin up to the ordinance of Villers-Cotterêts in 1539 which requires the keeping of registers in French and no longer in Latin.
But there are anomalies, as in Moselle where the registers remain written in Latin until 1750 in Rettel. The baptismal certificate of François André in January 1741 stipulates "[...] baptizatus est Franciscus filius legitimus Jacobi André and Margaritha Maller [...]"
View 84/91 left page last paragraph
In the French used in acts under the old regime, we find a few old verbs such as “inbury” (bury a deceased person) which reminds us of verbs created in French-speaking African countries such as “to camembert” (smell strong from feet) in Senegal.
Inbury is the verb used for the funeral of Pierre Biet in September 1612 in Gévezé in Ille et Vilaine. Right page 2nd paragraph

Good and bad pupils

Under the old regime, the registers are written more or less well, depending on the goodwill of the parish priest.

Bad pupil

Here is an example that could be described as "doctor's writing" in Hédé (Ille et Vilaine) in 1618:

Good pupil

On the other hand, at La Ferté-Milon in 1721, everything is clear and legible: