RomualdBuaud Do you at least know how composer work ?
If no, you must first learn that. https://getcomposer.org/doc/00-intro.md
Instructions are clear wether you are french or not. If you cannot read in english, mighty google translate (or another translator) can help you.
On how to put a language pack on Flarum (the french one), it's really clear. You just have to execute a composer command.