Version 1.3.9

Try changing line 976 (version 1.3.9h) in includes/modules/payment/paypal/paypalwpp.php and change true to false.

The line you are looking for is:

PHP Code:
$storeISO = zen_get_countries(STORE_COUNTRY, true); 
which should be changed into:

PHP Code:
$storeISO = zen_get_countries(STORE_COUNTRY, false); 
Now Paypal (Express) should be directing your users based on either the default language or, if enabled, the language preference as set by the user.