If you don't find any phrase in .PO or .MO file you should update that files using Poedit software. All phrases exist in POT file, you just need to update the PO file. Just open the PO file using the Poedit software and click on update button, then save it.

You can also translate that files using wpForo Phrase system. Navigate to Dashboard > Forums > Phrases admin page and search for that phrases, then edit and change the English phrase to french. If you don't find that phrase, click on Add New button, fill the Phrase key with identical English version, and value with French version, then save it.