Ah, on risquait pas de trouver! La langue anglaise est pleine de surprises!
(en gros des Groupes Enfants se dit “group child” :)

the label there is for “a child object of a group” or “group child”. That’s for a single one. For multiple of that in English it would be “group children”.
That comes from these entries in the strings file:

Properties panel detailed type 1 group child=1 group child
Properties panel detailed type N group children=%1% group children

If you want the French version to say “3 groupes enfants” where there is an “s” added to both words you can do that if you want in FrenchStrings.txt .
FrenchStrings.txt is currently using these values that you set:

Properties panel detailed type 1 group child=1 group enfant
Properties panel detailed type N group children=%1% groupe enfants

If it’s not right you can change it there in FrenchStrings.txt .

Je vais faire des essais et voir si ça marche mieux! :)