I think I was a bit unclear last time, I just had another look at that topic to see what I said.
You do need to add them back to the languages folder after the theme is updated, I just meant that you keep them somewhere and they are not changed, not that the update doesn’t remove them. Sorry about the confusion.
If the translation file is without any strings specific to your website (meaning: just a regular translation from English to French) you can send them to me and I can add them to the theme and you won’t have to worry about them anymore. This way others can use them too.