You can use localectl to change the locale on Fedora. Here’s what you need to do:

• See if you have Japanese locale installed. Something like ja_JP.UTF-8 should be in the output of localectl list-locales.
• If it’s not, you should install it using the following command: sudo dnf install langpacks-ja (I’m not 100 % sure about this and I don’t have a Fedora system to test it on.)
• Set the locale: sudo localectl set-locale LANG=ja_JP.UTF-8
• Reboot your system. Everything should be in Japanese now.

This will (probably) change everything to Japanese – texts in menus, error messages in the terminal, and also the font rendering. This answer on Stack Overflow suggests to do something with your fonts.conf. This way your UI would be in English (or your preferred language) and kanji would render as the Japanese variants.

A. I don’t know much about CJK fonts. I’m just spitballing. I am also half asleep.

B. It depends where the font is displayed. As you probably know, different Japanese, Korean and Chinese characters, which share history and look similar, share one unicode codepoint, see this Wikipedia article. Which specific glyph is shown is decided by some variable that specifies in what language the text is written:

• If the text is somewhere in the GUI (the title bar, the panel, some menu), it is probably decided by your default language and locale. This can be changed somewhere in settings. Changing this would also probably change everything to Japanese.
• If the text is somewhere on the web, this is decided by the lang parameter of the website. You can’t change this easily.