Hello @chokri
To change your locale go toadmin panel and click on Settings >> Locales >> Create Locale as shown below
DFGB.png
now after creating your French locale you have to assign it in the channels by clicking on settings >> channels >> edit channel >> locales & currencies
Now enable your French locale and enable your default locale as shown in the image.
stdhn.png
You can also took a reference from the User Documentation below.
Link:- https://docs.bagisto.com/2.2.0/settings/locale.html
Thanks & Regards