Good evening Thierry and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the updates overwriting your translations.
I would love to help you with this topic and offer to continue the conversation in French if you prefer.
The good news is that we will soon add a feature where you can simply store your customized translations in your /wp-content/languages/ folder and not worry about that. Unfortunately, I cannot commit to a release date at this point. But stay tuned!
In the meantime, you have a couple of options available for you:
Let me know if that helps.
Have a great day!
Geoff B.