I don't think this is a browser problem or game problem, itch.io seems to only support common files. Adding non UTF-8 Encoded characters like
ひほわれよう
or
നമുക്ക് തകർക്കാം
you'll get error. what you want to do is change anything that is programming into the game. This means the programming has to be english or close to the english wording, i know i don't like using it too because i don't understand much either but it should fix the problem. Unless it's inside textbox then i have no idea how to fix it.