I am glad that Google Docs at least recognizes and displays Khmer text now. Before, the script wouldn't appear at all. However, is there a way to select international fonts, like Khmer, so that Google Docs can effectively be used for non-English scripts in which special fonts are needed?
Yes! Google has an extensive collection of webfonts for webapps hosted on its Google Fonts site, including (currently) 23 different Khmer fonts. (On the left side of the directory, just change the "Script" option to Khmer.)
To add Google Fonts to Google Docs, you will need the Extensis Fonts add-on. Once you have installed the add-on, open a document in Docs and launch the add-on. Find the font(s) you want to add to Docs on the Google Fonts site, then go back to your document and search for the font's name using your browser's text-search function to locate it in the list of fonts. Click it to add!