weave
Start a book in English and finish it in another language, with the words swapping over as you learn them.
Working, with an ingest pipeline for real books and a built lexicon.
Weaving, gated by what you actually know
A diglot-weave reader swaps individual words of a text into the language you are learning, so the sentence around each new word carries you through it. Done naively it degrades into a wall of words you have never seen.
Here a word is only ever woven in after you have met it as a flashcard. Each chapter opens with roughly fifteen of its most useful words, most frequent first, scheduled by the same spaced-repetition algorithm Anki uses; the chapter unlocks for reading when those cards are done. The text drifts towards the target language at exactly the rate you can follow it.