To install Khmer keyboard is not kinda hard but you can't find it via the UI. To add Khmer keyboard you have to follow below:
- Go to start => Setting => Keyboard
- Click Layout tab
- uncheck "Use System deault"
- click "Add" button
- choose "Khmer (Cambodia)"
- Click "OK"
- open termianal and type command
setxkbmap -option grp:switch,grp:alt_shift_toggle us,kh
Try to switch keyboard via "Alt+Shift" then you can type khmer.
