How to translate Booking Calendar plugin
https://wordpress.org/plugins/booking/
To translate the labels of the form fields, you need to enter the Field Labels like this:
Booking > Settings > Form
Example:
First Name[lang=nl_NL]Voornaam[lang=it_IT]Nome di battesimo[lang=fr_FR]Prénom
Last Name[lang=nl_NL]Achternaam[lang=it_IT]Cognome[lang=fr_FR]Nom de famille
etc.
If the Booking Calendar refuses to load on your translated pages, then check this setting in Booking Calendar:
> Booking > Settings > General > Calendar > Show advanced settings of JavaScript loading
If you have set "Load JS and CSS files only on specific pages" then you need to add the specific translated pages in the fields. Or alternatively you can uncheck this option.