Hi, I’m using the latest version of Budgie on Mint 20.2.
I saw in Settings > Region & Language, that there is an option to allow different sources for each window. I checked and enabled this option but it does not work even after restarting. what could be the problem? I would be happy to provide more details if required.
$ gsettings get org.gnome.desktop.input-sources per-window
» true
It’s not a Budgie issue, the option worked on and off over different Ubuntu releases. I suggest using the keyboard autoswitch applet for now.
The applet/extra does not work either. but thanks for let me know that problem is global 
Could you tell what happens/how you are using it? With the application in front, just set another input source, it is then remembered for that application.