Change the default keyboard input when logging into macOS Big Sur

For your consideration:

1. Start by going to System Preferences: Keyboard: Input Sources, and make sure your preferred language is included in the list. Remove any unwanted languages from the list.

2. Go to System Preferences: Users and Groups: Login Options. Unlock the preference pane if needed.

3. Toggle the “Show Input menu in login window” option off and on again. (Toggle it on if it wasn’t already enabled.)

4. Go to System Preferences: Language and Region: General: Preferred languages.

5. Temporarily add a second language to the list using the Plus button. You’ll remove it later so just add any language.

6. Drag the new language to the top of the list.

7. Click the new Gear button that appeared on the bottom bar when you rearranged the language list, and choose Apply To Login Window. (As shown in the above screenshot.)

8. Drag your preferred language back to the top of the list again.

9. Repeat step

10. Remove the language you temporary added to the list by selecting it and clicking on the Minus button.

11. Reboot your Mac.

For me the one that actually worked was:

1. First change completely Mac OS X system language (in Language & Region) to desired keyboard language layout. In my case it was Finnish (even though I use English as my Mac OS X preferred language normally)
2. Make sure that you have in your (Keyboard -> Input Sources) your desired input language as first option (in my case it was Finnish)
3. Then reboot your computer
4. After reboot, run: sudo cp ~/Library/Preferences/com.apple.HIToolbox.plist /Library/Preferences/
5. Change system language back to the desired one (in my case English)
6. Reboot your computer once again