https://readini.com
Side by side, synchronized audio, dictionary, learning words.
I'm the author. Considering coming back to working on this project, with more books and languages.
I am interested in English-French at the moment and was hoping to use sideloaded ebooks and mp3 audiobook versions - not necessarily just old classics out of copyright.
It seems there are some open source projects that replicate Amazon Whispersync (sync audio to text and highlight words/phrases as they are spoken).