The correct taxation of your customer is determined by the billing address of the customer. The country stored there is compared with the tax rules stored in billwerk and thus a customer with a German billing address receives the tax rate configured for Germany. If the customer's country is not explicitly defined as the tax rate, the system checks whether the country is in the European Union. If this is the case, the tax rate configured there is used. If the country is outside the European Union, the tax rate for the "Rest of the World" is used. You can find more information on the tax rules here.
If you want to set or change the customer's country via REST API, you have to fill in the value "Country" in the customer object. The two-digit ISO 3166-1 code is used for the country codes. A list can be found here: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2