You will need to install a user script manager extension to install this script.
It is exactly similar to https://greasyfork.org/en/scripts/439780-wanikani-community-prioritize-japanese-glyphs but add more websites.
This script may also be edited to prioritize other langs, like "zh-TW" or "ko-KR", if you really need it.
It is possible to edit the UserScript to add more websites, for example
// @match https://www.notion.so/*
The script is based on lang attribute, and no CSS is really being injected. (I might add CSS injection, based on common fonts, later.)