1- You just copy  khmer Unicode in to ur SDcard then run CWM manager      and

 

2- get reboot in CWM then click back button till your phone show like below

3- then you chose  install zip from sdcard

4-  then choose Choose zip from internal sdcard.
5- so you find ur zip file then choose YES
6- after that you back to reboot ...till 
7- if you want switch Keyboard pls use MultiLing Keyboard download from Google Play 
8- enjoy

Font khmer Unicode for  SonyEricsson Xperia Arc S (ICS 4.0.3)

http://www.mediafire.com/?7rrqu83ystuhb3e

Clock Work Mode manager 

http://www.mediafire.com/?49awagaphz2xa93
http://www.clockworkmod.com/rommanager

NOTE: Your phone need to Root and unlookbootloader ,this Font khmer Unicode not support all phone !!
Warning : it may lost use ur system !!!