Hi Kees,
It is the English as the second language, and of course the Serbian is the first.
There is a somewhat obscure presentation on the site about ordering at
http://www.wingsofserbia.com/order
but it should be simple, at least for people having a Paypal. An e-mail at the provided address
book#wingsofserbia.com
(change # for @) is enough for the details about the Paypal payment.
The bank wire transfer is another option but it is a bit more complicated as it would require IBAN, SWIFT etc. for each particular country of destination of the book.
As for the payment with VISA, Mastercard etc. this is unfortunately not possible at the moment as the publishing house is a very small one and does not have the necessary permissions for such transactions.
Cheers,
Boris