Sorry for that. Probably its because in the Danish translation not all the words are translated. Please check how to check it and fix it here.
Also please note, if you are use the free version, after fixing the language in the translation file, you are need to make the full reinstall of the plugin - its mean that all booking data will be removed (reseted).
If you want to proceed with that, so please open firstly the General Booking Settings menu page of Booking Calendar admin panel, and at the right side of the page recheck that this option “Delete booking data during uninstall” is turned On (checked), it's mean that all your booking data during deactivation will be deleted.
Open the WordPress Plugins menu page and click on “Deactivate” link of Booking Calendar (free version). Wait for the message “Plugin is deactivated successfully”. It’s important.
Open the WordPress Plugins menu page and click on “Activate” link.
Wait for the message “Plugin is activated successfully”. It’s important. Otherwise not all DB tables will be created, and you will have an error messages.
test it now.