KiCad translations are managed by Weblate. The Weblate project is periodically synchronized with the KiCad source code repository on GitLab to apply newly-created translations and to import new or changed source (English) text into Weblate so that it can be translated.
Please see the Weblate documentation for details on how to use the Weblate interface to translate KiCad.
At this time, the French translation of KiCad is managed outside of Weblate by Jean-Pierre Charras, so French is not available as one of the translated languages to edit in Weblate.
Please use the developers email list for any questions or issues relating to translating KiCad.