To help everyone play this mod better, I have created a Japanese translation of ja_jp.json and Chronicle of Shadows.
I was able to send it from “Create new file” as ja_jp.json, but Chronicle of Shadows is a folder, not a file, so I couldn't figure out how to send it.
Does anyone know how to send it?

You can submit the change as a pull request to the repository. If you want to package the localization externally, you can bundle it as a resource pack:

apoth_jp.zip
--pack.mcmeta
--assets
----apotheosis
------patchouli_books
--------apoth_chronicle
----------ja_jp (your translated files live in this folder)
------------categories
------------entries

I've pulled the changes from your fork branches into the 1.20 branch here 83d50de
1.21 will need to be updated separately, I'm afraid the language files are not compatible after the update and I don't have the means to update these ones myself.