Here's how to add a customer:

1. Go to Customers and open "New Customer"
2. Enter the customer or company name and contact details
3. Add the email, phone number, and address fields you want to keep on file
4. Fill in VAT number, IBAN, and payment terms if those apply
5. Save the customer

Tip: The customer form does not have a separate preferred-language field right now, so focus on the billing and payment fields the app reuses later.