A form you have already created can have the locale changed which would change formatting of time/currency, spell checking etc, but not the english text to spanish text. Using the Templates on the "Create New Form" tab, you can change the language to Spanish (language selector at bottom left) and all Templates will be Spanish, you can then create a new form from any of them in Spanish. Let me know if I can provide any more details.
FormsCentral does not have a means for translating a form from one language to another. You can, however, use Templates to create a new form in one of the supported languages. To do this you go to the Create New Form tab. In the bottom left of the page there is a drop down selector that lets you select the language. After selecting which template best fits your needs you select Español from the drop down. You then click on the Use Template button on the Template preview that you wish to use. The new form will be in Español. I hope this helps.