Ask our support team to turn on Detailed Booker Info for you on your account. You can then open a reservation and click on the green EDIT button under their name:
You will then be able to add your Lead Booker's Nationality from the drop down provided and then click the UPDATE CONTACT DETAILS.
If you need to create a report for the Nationality of Guests Staying - then head to INSIGHTS > CSV REPORTS > CUSTOMERS and download this for the dates of Stay.