Salon displays in English only

Hi,
The Salon plugin displays only in English, while the site is in French (and English also using Polylang).

When activating the debug, the following PHP message appears on the page when the plugin should appear on the site:
Notice: Undefined index: default_locale in /…/www/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 94

And when going to the admin section of Salon, the following messages appears (debug still active):
Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 101

Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 101

Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 103

Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 101

Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 101

Notice: Undefined variable: locale in /…/wp-content/plugins/salon-booking-system/src/SLN/Helper/Multilingual.php on line 103

So what’s wrong? ??

Thanks for the assistance,
Arnaud