Unfortunately this is a tricky edge case that I don't quite see a clear path to resolve centrally. CommCare doesn't 'force' the default language configuration after initial install since updating the app language on the device directly is the most common user configured parameter.
The only current long-term way to address that I can think of would be to manually update the language from the mobile app.
I have another, hacky idea for you, but it might have challenging side effects so I'd carefully check how it works:
If you change the language code of the english translation to "zxx," I believe the app will default to the top language in the list when it updates to look for another language.
This would really only work if you didn't need the english version to ever have the right tag, though. The app will still think the 'right' default key is still "en," so if it ever gets reverted back to the 'right' key, the users will have the app switch back on them.
If the english translations are simply for reference, and french is the only "production" language that will need to be used, I think that hack will prevent the need to manually update each phone.