Front end labels of Appointment+ in French

Hello,

this is Stephane again from Paris France.
I installed Appointments+ on the following site :
http://www.coquettissime.fr/reserver-votre-beauty-party-maintenant/
I would like to translate all the front end labels in French since this is for French clients.
I started to edit the default.po language file in /wp-content/plugins/appointments/languages and uploaded in the directory my edited fr_FR.po and fr_FR.mo files.
However, for the fields/labels that I have already translated, I cannot see any difference, all the labels are stil in English.
In my wp-config.php file I have set :
define('WPLANG', 'fr_FR');
What am I missing or doing wrong?
Thank you for your help to tell me what i need to do to have the front end labels translated in French for Appointments+.
Cheers,
Stephane