Hello.
Booking Calendar is support multilingual, in case, if you are using the WPML or qTranslate plugins for setting different languages at your website. Please check more here: https://wpbookingcalendar.com/faq/several-languages/
The language of calendar, have to be switched automatically depend from your active locale of website, which is defined by those translation plugins.
The Emails or labels of booking form in Booking Calendar free version at Booking > Settings > Form page also possible to translate relative to previous instruction from plugin FAQ. Example of label configuration in 2 languages:
First Name[lang=fr_FR]Prénom
Here is one exception, if you are using WPML plugin together with Booking Calendar Free version, so then booking form labels defined as translation terms in WPML plugin and you need to translate them in usual way at WPML plugin. WPML generated custom fields for those texts. Once you have translated this items it will go well.
Kind Regards.