Not exactly Duolingo alternative, but: I'm currently building an an app for language learning by reading books. It takes an (your) ebook and inserts translations:

—Arthur —dijo con tono cortante, ["Arthur," he said sharply.] y su voz sonó como el chasquido de una ratonera—, [and his voice sounded like the click of a mousetrap.]

There is zero gamification, as it's not needed - you're motivated by the pleasure of reading.

It won't help you to learn to speak (there is an option to read aloud a selected phrase though), but it will help you with vocabulary. I went with it from not being able to read even one page, to being able to understand 70% of a book (intermediate Spanish level) in about a month.

ATM looking for beta users, completely free of charge. Pls leave a contact in a comment if you're interested