1. Install Fcitx 5 flatpak version
  2. Install Chinese Addons flatpak version
  3. Add Fcitx 5 to Auto Start on Boot in System Settings
  4. Start Fcitx 5 -> Configure -> Add a Chinese input method (e.g. 双拼) -> Confirm
  5. Add the following to /etc/environment:
GTK_IM_MODULE=fcitx
QT_IM_MODULE=fcitx
XMODIFIERS=@im=fcitx