fonts/etc_fonts/conf.avail/65-khmer.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
	<alias>
		<family>serif</family>
		<prefer>
			<family>Khmer OS"</family>
		</prefer>
	</alias>
	<alias>
		<family>sans-serif</family>
		<prefer>
			<family>Khmer OS"</family>
		</prefer>
	</alias>
</fontconfig>