I built DuoText for myself. I noticed I get good results learning German with parallel texts, but there wasn’t enough parallel material. I also wanted to read my own books that way, so DuoText started as a tool to turn any book into a parallel-text edition: the original and a translation side by side, paragraph by paragraph.
Then I used it to build a library of stories in 17 languages, in any pair. Reading the library and tapping a word for its meaning works in the browser; a free account adds downloads (PDF, Kindle/EPUB). Translating your own books, and some of the stories, need a subscription.
The translations are machine-made. I’m still working on it, so any feedback helps.