Currently Ubuntu 11.x does not allow for all Khmer vowels to be correctly typed.  If you’ve had trouble typing Khmer in Ubuntu, try this solution and see if it works – it should allow the NiDA keyboard that comes with Ubuntu to be fully used.

Here’s how it can be fixed:
1. Type
sudo gedit /etc/environment
2. Add a line
GTK_IM_MODULE=xim
3. Save and close.

4.Then in terminal type:
sudo gedit /usr/share/X11/locale/en_US.UTF-8/Compose

5. Gedit should open, search for Khmer and make sure the following exists (if not, copy and paste it in):
#
# Khmer digraphs
#
<U17ff> : “ាំ”
<U17fe> : “ោះ”
<U17fd> : “េះ”
<U17fc> : “ុំ”
<U17fb> : “ុះ”
6. Save and reboot.

And that’s it!  Let us know if you have any trouble in the comments.