MacOS – How to fix macOS Sierra language switching bug

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.

The command as well as the shift key (and some other keys) are modifier keys. You have to add at least one normal key (like A-Z, 0-9, Fn*, arrows etc) to define a proper shortcut.

If you want to change this behavior you have to install a third-party tool like Karabiner.