Freeing the system shortcut

While Langouste is active it takes the keys you recorded. If macOS has the same keys for its own input source switch, turn that one off so the two do not fight.

1. Open System Settings › Keyboard › Keyboard Shortcuts › Input Sources.
2. Turn off “Select the previous input source” if it uses the keys you gave to Langouste.

Switch Langouste off (the Active item in its menu) and the system shortcuts work as before.