Mine still wasn’t showing, but I noticed in this post that some fonts you need to just type it in and hope it works (rather than relying on their autocompletion).
Trying that worked for me, at least for the Khmer font:
As I was typing, it doesn’t recognize the font and give the little red circle guy:
But when I finish typing the full name out, it seems to recognize it:
In the end, what’s important is that it works 
Before:
Result:
