I did what you advised me @robert, but that does not solve the problem, cache emptied, I even waited several days to be sure that it did not come from a cache...
the second is no longer in the database, I can no longer find it in the list of phrases, but it still appears?
Ok, it seems the phrase is also translated in the language PO file. Install Loco Translate plugin, and, edit your language file and search for that phrase. If it's translated just change it back to the english version.
I forgot to specify it, but I had already checked if it was present in the translation file. I just rechecked the PO file (with Poedit).
While writing this message, I searched again and...
I found the problem!
I had checked the PO file in
/wp-content/plugins/wpforo/languages/wpforo-fr_FR.po
but the problem came from this file
/wp-content/languages/plugins/wpforo-fr_FR.po
Well, maybe my mistakes will help other people.
Thanks again to you @robert for guiding me and the time spent helping me to the end!