Hi, here are some answers.
1. The most flexible way to design the look of the Chapter Component is to use the “Overlay” field under Chapter component options. You can use HTML and style tags here. And it will replace the contents of the Title and Subtitle fields. (You still need to fill out the Title field for the chapter index) So you can use other fields to design the look of the background and use the overlay field to design the look of the text. You can use color, font size and what not to design the look of the text however you want.
2. That “Chapter” text is coming from the Longform theme, not Aesop, so you need to change the Longform Theme. For your purpose, the best way to do would be to use language customization. Here is the link you should refer to:
Longform is translation ready. So you should take the English PO file and create the Spanish language files.