It seems that ArcGIS dees not support Khmer Unicode even its latest stable version 10.0 while most of the program.
However, ArcMap 10.1 has been significantly improve to support such complex characters although there are still some wrong spaces and overlapping characters.
The is strange is all parts of ArcMap's interface (such as Table of Content, Attribute Table,...) could properly display those Khmer words and characters except for the Map Display Area. Why could this happen in the same application? How to make Map Display Area shows Khmer Unicode characters properly like its other parts?