Install fcitx5 with the following commands:

sudo pacman -S fcitx5-im 
sudo pacman -S fcitx5-chinese-addons  fcitx5-rime

Add the following config lines into /etc/environment:

# /etc/environment
GTK_IM_MODULE=fcitx
QT_IM_MODULE=fcitx
XMODIFIERS=@im=fcitx
SDL_IM_MODULE=fcitx

Reboot the system.