Is there anything (addon..) to turn the frontend to French?

Yes, you can install French translation (not 100% completed though, but anyone can contribute to it!).

• https://github.com/pH7Software/pH7-Internationalization#available-languages
• https://github.com/pH7Software/pH7-Internationalization#how-to-install-those-languages
• https://ph7cms.com/doc/en/how-to-translate-to-another-language