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.