You'll want to look into our localization and messaging options for this. If you need the ability to switch between english and spanish, then you can facilitate that using our localization tools (and can set the default language as spanish.) Basically, you use the localization and messaging editor to create message objects which you then reference in code wherever the user sees visible text.
If you only need to deploy in spanish (no english required), then you could always just develop the visual aspects of the application (lables, form titles, etc.) in spanish and deploy that way, but we still recommend using the messaging editor to do so just as best practices. Keeping all your messages centralized just makes maintenance easier down the road, and makes it a lot easier to come back and impliment other languages if the need arises.
The help files are pretty extensive on leading you down the path for the localization and messaging, but we'd be happy to give you a point in the right direction or help you work through any issues if you have any specific problems or questions!
Just let us know