After upgrading to macOS High Sierra, my Japanese and Chinese input methods disappeared How do I get them back? They're neither in my Input Sources, nor can I find them if I try to add them.
Reboot into Safe Mode (hold Shift while restarting). You can then log in (it will be slower than normal) and add your input methods again. Reboot again and the changes will have persisted and you should have your Input Sources back.