My only serious problem with macOS is keyboard language when switching between apps. It changes the current app's language to the one used by the previous app.

My only serious problem with macOS is keyboard language when switching between apps. It changes the current app's language to the one used by the previous app. Reported it countless times, had a few email exchanges with Craig Federighi, still unsolved after 13 years. Can anyone write an app to keep each app's input language? I think the OS part that deals with that is still Carbon...