1- 404 issue: I implemented the PDF generation recently and I noticed that WeasyPrint has issue with html files that have too many tags (our books have around 2*number_of_words tags in them). This is not a big issue and it will be fixed soon in the next iteration.

2- Using Google API: Google APIs and other translation tools are great for translating sentences. However, the problem with use of parallel texts for language learning is our brain laziness. After few pages, our brain looses its patient to solve the translation problems (critical thinking!?) and actually learn words and structure of sentences. The focus immediately goes toward translated sentences in your native language rather than the original text.

Personally, I learn a word for a life when I slow down and think about similar words, its root, and at the end looking it up in a dictionary. The process is valuable.

3- Team of volunteers: It is easier said than done. The functionality is present but I prefer to improve the suggestion engine as much as possible before I involve volunteers. Are you interested to join?

I prefer https://www.deepl.com/translator to Google.

'hermanos' is translated 'brethren' - super-archaic.

p8, 'dedicado' in 'te has dedicado a pintar?' is translated 'hardcore'.

'sí' in 'Que sí, hombre' is translated 'do', as in do-re-mi, I guess.

p5,11-12 has 'quieres/quiero repeatedly translated as "with friends like those who needs enemies". Which is just inexplicable. I can't imagine how that would happen.

Corrupted dictionary?

..and most of the trickiest words on a page aren't translated, maybe because not in your dictionary or they have 'lo' or 'se' appended.