There is no requirement to use English language buttons (you could use Spanish text or custom buttons), and as long as the product or service descriptions are in Spanish there should be no proble. PayPal accepts several different currencies. They are far less costly than StormPay, ClickBank, 2CheckOut and most others.

While WorldPay supports European Spanish, this is NOT the same as Mexican Spanish, and far from Brazilian Portugese, and the setup is much more difficult. With poor credit, PayPal is very definitely the best bet. None of the payment systems that I'm aware of support Mexican Pesos, except http://www.mexican-gold.com and I'm not sure how solid they are.