@pierre-ladonnewanadoo-fr: Hello there,
To translate the plugin into French first of all you will need to use Poedit software. Install and Open the software.
Create New Translation => Browse and Select catch-breadcrumb-pro.pot file from the languages folder and Click Open => Select the language as French (France).
Now, you will see a list of translation ready strings and Translation text area where you can add the required translation. Search for the strings you want to translate and add the translation. When you are done translating the strings save the file as catch-breadcrumb-pro-fr_FR.po to the languages folder.
Now,Go to => Dashboard => Settings => Select Site Language as Francais and Save Changes.
You should be to see the strings translated to french.
But the translated files when be lost when you update the plugin to newer version, so when you are done translating all the strings you can email us the translation file so that we can add the translation to the plugin folder with your name as the contributor.
Let me know if this helps you out!