All e-mails in same language

Hello.
1) Please check how to configure email in several languages here https://wpbookingcalendar.com/faq/several-languages/

2) But please note, before approving booking or cancellation or sending payment request you need to select specific language for email in the Booking Listing page by clicking on Globe icon near specific booking and then selecting the language (locale), and after this clicking on approve (or other button) for sending email in specific language. Watch it in this video guide https://wpbookingcalendar.com/help/booking-listing-video-tutorial/

If you need to show in what language was made the booking, so then please check this FAQ instruction: https://wpbookingcalendar.com/faq/get-page-active-locale/
Kind Regards.

Hello.
Sorry, its does not available in the Booking Calendar Free version.
In the Booking Calendar Free version the only way to send the email in specific language (during approving or cancellation of the booking), its to switch the language of the booking admin panel (usually its menu in top dashboard, if you are using some translation plugin), and only after this approve or cancel the booking.
Kind Regards.

Hello.
Unfortunately, the Polylang plugins does not change the locale of your admin panel.
As you can see even the WordPress menu in English and other Booking Calendar interface also in English.
You need to use some other translation plugin or contact support of Polylang plugin about ability to switch local of entire WordPress admin panel.
Kind Regards.