[Bug] Books do not use English translation when others are missing

[Pelemenguin]

Current book behaviour

The BookData loads language files in a book repository only for current language.
This causes that, when some of the translations in a language file is missing, the book directly use the raw translation key, instead of considering to find the ones in the en_us.lang.
(According to the source code, this bug still exists. (At least it exists in 1.20.1, 1.11.79))

Expected behaviour

When one translation key in a certain language is missing, the book should look it up in the en_us.lang. If still not found, use the raw translation key.

Possible solution

Let the book read en_us.lang at the same time when reloading resouce packs.

[KnightMiner]

I am not entirely sure this is viable due to how book loading works. The book specific lang files are loaded like any other book file, which does a per file language check for missing content rather than a per book check. I suppose the loader could just try loading English and then load the other language on top, but not sure it will handle that well.

I do overall keep trying to phase out those files since they are largely redundant to the normal en_us.json and alike. Though a couple book specific things are just way easier to not outsource, notably with a few of the optional translations.

Which notably is another argument towards keeping it from falling back to english as a lot of the optional translations fall back to something if absent as they are just used to override problematic things in specific languages, such as too long names. Different languages might have different problematic things.