You are not very helpful to people who want to help you.
First of all: there are two error logs where you can look. One is the error log of the (Apache?) server. The other is below the /var/logs directory of your shop.
Second this "spanish customers" is too vague. How many of them does it concern? Are you sure it is not some specific product that is ordered? How about their names and addresses: could it be that accented characters are causing problems? What happens when you make a test account with the same names, addresses, orders, etc? Is your contact with some of these customes good enough that you can ask them to cause the error while you have enabled debug ,ode for a short time? "try to checkout" is vague too: in which step does this error happen exactly?