Our internationalization features aren't very good at the moment. I'm afraid there's no option to change the default button text on slider questions.
There is a workaround for the specific case you mentioned – you could use an explicit
*page keyword and make sure you end that page with a
*button. You're free to use whatever text in whatever language for the button, and it'll look like a slider question:
*question: How are you feeling?
*button: Move on to the next page
In your case, you'd use Spanish for the question text and button.