plasma-workspace-5.20.1.1-1.1 cannot be installed with:
...
plasma-workspace: /usr/share/locale/zh_TW/LC_MESSAGES/kfontinst.mo exists in filesystem (owned by plasma-desktop)
plasma-workspace: /usr/share/locale/zh_TW/LC_MESSAGES/krdb.mo exists in filesystem (owned by plasma-desktop)
plasma-workspace: /usr/share/polkit-1/actions/org.kde.fontinst.policy exists in filesystem (owned by plasma-desktop)
use this to overwrite those duplicate files:
pacman -Sy --overwrite='*' plasma-workspace