The Danish translation is exist in the Booking Calendar (bit its only 38% Completed ).
Please read more at the FAQ at the translation section.
The booking calendar is support changing the interface based on locale of the site. So you are need to have the correct locale settings at the config.php file at the root of your site.
The Booking Calendar is use the active locale of the site for the activation of the specific language from the ../booking/languages/ folder.
For activating the specific translation you have to activate the locale at the config.php file at root of your WordPress installation.
If you are use the free version, so then you are need to make the FULL reinstall of the plugin, its because the email template and labels to the booking form is saved to the DB during activation of the plugin and you are need to make this reactivation. Its mean that the exist bookings and settings will be erased. Here is instruction how to check it or fix/add translations
If you are using the paid version of the Booking Calendar and need only the booking form labels at the client side of your site in different language, so you can customize the booking form at the Booking > Settings > Fields page at top form. Please test it at the live demos