Option1:- Write a small script to update all the existing group
Option2:- Create a parent group & set language to french & then make all other group member of it.
Option3:- In 5.4 configure preference locale for property security.preferences.defaultlanguage to fr. From 5.5 it should have been moved under granite find the respective configuration in felix console.
[1] http://<host>:<port>/system/console/configMgr/com.day.cq.preferences.impl.PreferencesLocaleResolver