Apple - Change shortcut to change input source (keyboard language)
Here are Apple's instructions for changing shortcuts found in PH21531:
Choose Apple menu > System Preferences, click Keyboard, then click Shortcuts.
Select the action (Input Sources in your case) in the list that you want to change.
Double-click the current shortcut, then press the new key combination you want to use.
You cannot use each type of key (for example, a letter key) more than once in a key combination.
- Quit and restart any apps you’re using for the new keyboard shortcut to take effect.