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To set the default language you should open your jotform and then click on the Languages option in the toolbar
When the window opens for languages you can set default and see what the default is. For your jotform I see that it is already set to Spanish.
Now your jotform is loading in English because not all strings (text) on your jotform is translated yet, only the header. My suggestion is to make all the translations that you might need and the jotform should load to you in Spanish once you go to it.
You can see more about the languages and how to use this feature when creating your jotforms here http://www.jotform.com/help/298-How-to-make-your-forms-multilingual
If you need any further assistance feel free to contact us back :)