So if I try to switch keyboard to Russian Language, this book will freeze the game forever. So there is no crash-report. Logs also tell nothing.

I'll take a look into it. I haven't tested the mod in anything but English, but it might be an easy fix.

The issue is due to the font renderer I use to create custom font. It's getting stuck in an infinite loop because it doesn't know how to draw the Russian character. I will try to figure out a way to ensure it creates the correct (Based on keyboard) language characters and draw a ☐ if the letter is unrecognized.

Fixed in version 1.2. You'll need to enable "Use Calibri Font" in the afraidofthedark.cfg file to show Russian characters: https://www.curseforge.com/minecraft/mc-mods/afraid-of-the-dark/files/2887243