You might find interesting some ideas behind EPUB3Reader, an experimental EPUB reader (for Android) designed for handling complex texts, including parallel texts:
https://www.mobileread.com/forums/sho...d.php?t=222054
I just want to add to the discussion: today's methods rely on
graphical principles (e.g., devising two side-by-side columns of text, or interleaving paragraphs, etc.) which in turns influence the actual code of ebook. Instead, I am a big fan of a
declarative approach (e.g., marking corresponding text fragments), as it will allow different reading modalities. For example, while a beginner probably needs to have original+translation in parallel, an advanced reader might just want to consult the translation only from time to time (e.g., via floating popups or panels at the bottom of the screen).
Please see
https://github.com/pettarin/epub3reader for more ideas on the above issue.