Here is okay:

Code of this page: https://github.com/PanSzelescik/coolgadgets/blob/master/1.12/src/main/resources/assets/coolgadgets/patchouli_books/book/pl_pl/entries/items/calculator.json

Minecraft: 1.12.2
Forge: 2772
Patchouli: 1.0-6

In my opinion, yes

Give this a try after the next version and let me know if it works better then.

Tested with 1.0-7.12 from maven. The same situation

Special characters should work, even Chinese has been tested. Are you sure the file is saved as UTF-8?

It does work for me:

The file as downloaded from github is indeed UTF-8, so not sure what's causing it. Maven might not have the very latest version, you may want to check again next release.

I propose keeping this open until validated as fixed.

I can't build Patchouli from GitHub, this package: https://github.com/Vazkii/Patchouli/blob/master/src/main/java/vazkii/patchouli/client/book/BookContents.java#L31 doesn't exist

I removed this import because is unused, and I can now build. It's working :) thanks ;)

I redownloaded all my files from GitHub, built and situation is the same. I am wondering what you using for editing text? (Notepad++, Notepad from Windows or other?)

We believe the issue has been found and would be platform-dependent. I will implement the fix this evening and will let you know when it should be fixed.

This should be fixed now; can you check this Pan?