Google translate now supports Khmer!  Check it out here: http://translate.google.com

Currently it is only in Alpha which means there are still problems with the translations. But you can help make it better!  Read more here: https://support.google.com/translate/toolkit/answer/147844?hl=en

Also, here is another article in Khmer with helpful information: http://it4ug.net/2013/04/19/improve-khmer-google-translate/