It was an error in the code
In the meantime I have solved the problem. I checked the code and found it strange that the country did not have a delivery_ in front like all the other variables, which are then passed to PayPal. I assume this was an error in the code. As soon as I have fixed it and placed for this a delivery_ in front of the country it worked.
But still the selection of the countries does not get populated from the billing address. Are there any solutions??