Open Cydia and tap on

Manage
Sources
Edit
Add



Write this address in the pop up box
reak.host.org/bt/
Tap on Add Source
Now just wait for Cydia to update the source
             

After done, go open Reak sources as indicated by the Khmer flag inside apply logo (see picture on the left)
tap on "iOS 5.1.1 Khmer Fonts" to install khmer font (see picture on the right)
after done, you will have to respring iPhone then go back to cydia
tap on the second one which is "iOS 5.1.1 khmer iPhon..." to install khmer keyboard
after done, your iPhone will respring
Now go to Settings
General
Keyboard
International Keyboards
Add New Keyboard...
Now go all the way to the bottom and tap on ក្ដារចុចខ្មែរ
Now test out your khmer keyboard and font
Enjoy!

***Special thanks to the creator of reak.host.org for his hard work for khmer community***