keyboardlanguage
Using Macbook Air M1. The keyboard Fn (Function) key has a globe icon to switch between languages. It works but a bit quirky. When it's not working, it tries to switch language but couldn't, and when I switch screen, the language switching dialog keeps popping up.
To make the keyboard Fn key work again in switching language, I have to restart my computer. See video attached – the Fn key is not even pressed.
I had this problem and fixed it by: Control Panel -> Regional Settings -> Keyboard and Languages (tab) -> Change keyboards (button) -> Selelct the top language added: (English United States) -> then click "Add" -> Navigate between the options to (English United States) -> click on "keyboard" -> now check the option "US"
This solved my problem, I hope it solves yours
Could it be that you are accidentally hitting the input source switching shortcuts? If so, you can disable them in the System Preferences.
Also, if you find that the input language changes when you switch apps, this is actually normal behavior. By choosing “Use the same one in all documents” in the input source options, you can greatly decrease the chances of this happening. (With the exception of a few apps that seem to ignore/override this setting. Very annoying.)
If you've experienced this problem only when using Japanese keyboards, then I guess there is a different cause. I use US keyboards, but this unintended switching happens to me too. Mostly when switching to Adobe apps.
Best Answer
Using Fn to switch keyboards is new with Big Sur. Buggy behavior has been reported by many users in the Apple discussion forums. Until Apple fixes it, probably best to use the traditional shortcuts found in System Preferences/Keyboard/Shortcuts.
I understand MacOS 11.2 should fix this problem when released.