Good evening Yiannis and welcome back!
Thank you for reaching out to us and trying to answer Gregory.
You make a great point, what is probably happening on your install is that the file located in \wp-content\languages\plugins\the-events-calendar-fr_FR.po is not up to date (with the latest strings).
Instead of deleting the file and losing your customizations, you could also update that file using the latest .pot file. You can accomplish this using Poedit and going for Catalog > Update from pot file.
Have a great day!
Geoff B.