Hi Mathieu,
Thanks for taking the time to submit this idea.
The Project Management Addon can be translated into new languages. Currently we distribute English, Hebrew and Spanish.

Whilst we aren't currently planning to increase the number of languages we maintenance and distribute ourselves, you can certain create one at your end by copying the /modules/addons/project_management/lang/english.php file and renaming it to french.php.
Then translate the strings within the file to French, and the translated text will be displayed.