How do you say AD in French?

Answer 1

AD can have two opposite meanings.
AD (Anno Domini, in other versions CE: Current Era, Common Era, Christian Era, or PD: Post Domino) is written "après Jésus-Christ" in French. That full version is most often abbreviated in "apr. JC" or "apr. J.-C.".
AD (as Ante Domino, meaning BC: Before Christ, or BCE: Before Current Era, Before Christian Era, Before Common Era) is "Avant Jésus-Christ", abbreviated as "av. JC" or "av. J.-C."

Answer 2

Google it or use a dictionary. It works for me and as long as you get the sentence structure right it makes no difference.