I am always familiar with using
Ctrl + Shift to switch between input modes (switch typing language).
The setting in
ibus-setup cannot let me choose this setting (or this very likely is my fault).
After reading this thread: https://forum.kde.org/viewtopic.php?f=17&t=162008.
All I have to do is run this command as my normal user and everything is fine now:
gsettings set org.freedesktop.ibus.general.hotkey triggers "['<Control>Shift_L']"
I personally thank FireHeadMan for his/her answer above.