Choose the default language that is selected when someone creates an appointment. By choosing the most popular language you can reduce potential for error if this field is overlooked.

I'll create a few languages by navigating to Settings => Languages and creating a few entries. Then I'll select a Default Language in Settings => Appointment Settings. Now that a default is selected when I create a new appointment I can see the default language is already chosen for me and I can change it from the default when necessary.