Lemapas,
You need to do a couple things:
Go to Administration> Agents and Software updates. Click on Check status now. You would have needed to enter a subscription ID already.
In the list of updates, you will see a listing for "Translation Assistant" and "Translations - French Translations". Download and install them both.
Next, Go to Dashboard> Quick Links> ClearPass Guest
Navigate to Customization> Translation Assistant. Click on the Option "
Go to Customization> Web Login and you should be Able to Choose French..
The Labels will be in English and you can change those like you did already, but the System Messages will be in French..