We have some code to set PayPal’s “LOCALECODE” to FR if the language in the order is set to French. The initial URL’s always the same, but I think PayPal sets the customer’s redirect based on the LOCALECODE that’s sent.
What are your shipping settings on your CartThrob->Payments->PayPal page?
I did some testing and it looks like you need to have it set to “Let PayPal handle shipping addresses” for the language variable and LOCALECODE to have an affect. Otherwise, the redirect appears to be based on your shipping country code.