I edited the UserScript a little to add more websites, not exactly sure whether this is an Arch Linux-specific issue. I also now realized that UserScript can be installed on Android.
Some websites to test (with 誤解).
// @match https://www.kanjipedia.jp/*
// @match https://youglish.com/pronounce/*/japanese?
// @match https://*.immersionkit.com/*
Yeah, ImmersionKit too, @mathewthe2.
On Android (which should be standard-compliant with <html lang="ja">), Goo, Weblio, ALC, Yourei are OK; but the rest (Kanjipedia too) still have Chinese counterparts without the script.
It is possible to edit the UserScript to add more websites, as well as use regular expression, if @match isn’t sufficient.