Hi Marie-France,
Thank you for this kind offer! Translations for plugins are always helpful.
I've never personally translated a plugin but as far as I know the information in this doc page should be up to date https://docs.moodle.org/dev/Translating_plugins
You can find the link to contribute to the language pack for the face-to-face module here: https://moodle.org/plugins/view/mod_facetoface at the bottom of the useful links section. We can look at how to merge this with the included French language pack at some point as it would be useful to be up to date in the default download.
Let me know if you have any difficulties,
Stacey