Reading abroad with the file you already bought
You bought the English edition while living in London. Now you are in São Paulo or Madrid and want the same book in the language you speak at home. The storefront shows a different catalog; your DRM-free EPUB is still sitting in Downloads.
Translating that file keeps your purchase useful. You are not pirating a new edition — you are reading the copy you own in another language.
Upload on the app, or start from a title page like The Catcher in the Rye in Spanish.