Thank you for your message. I'm actually going to pass this to English to French, as I'm not sure témoin should be the translation for groomsman.
@DearPrudence, what do you think? At UK weddings, the witnesses fulfil a legal function, I assume it's the same for témoins in France. They are the people who sign the register to say they have legally witnessed the wedding. It's possible that a groomsman might be a witness too, but equally possible that he might not. It's basically the male equivalent of a bridesmaid. He's there to help the groom get organised, make sure everything runs smoothly.
I think the problem is that French weddings differ from American weddings.
To me, "demoiselles d'honneur" and "garçons d'honneur" are not adults, but children. But maybe I'm wrong? @Lacuzon
Les Groomsmen sont les garçons d’honneur aux USA. Cette tradition est très peu répandue en France mais aux Etats-Unis, les frères, cousins ou meilleurs amis du marié sont souvent choisis pour être garçon d’honneur et faire partie du cortège. Ils tiennent le même rôle que les demoiselles d’honneur mais pour le marié.
I am not sure a French person who's not familiar with American series would quite understand what a "garçon d'honneur" is and how important he is, while "témoin" seemed like a close equivalent.
Now I realize that a groomsman might not be that important and might not deserve the status of "témoin".
I guess I'll change the translation to "garçon d'honneur" and add a note.