Few Spanish speakers search in Spanish for prepaid cards. The ones that do often type tarjeta prepago but come up with a bunch of prepaid phone card sites. If they want to find a tarjeta prepago MasterCard they have to type that phrase into the Google, Yahoo Search, or Bing. Or they can type Prepagada MasterCard.
Once they find what they are looking for, the site may not be in Spanish. Hopefully, the can read English. If they can't the best prepaid card for Spanish speakers is offered by MiCash.