Hello B Seng Kea
I just found a way that we can use Khmer fonts in Affinity but we need to download a free app called Inkscape. It's free and open source. All you need to do is just write the text and edit in that app and then you need to select Path>Object to Path then save it as SVG then you can insert that SVG file to Affinity Designer without error because that text file was converted to an object file. It available in MacOS too but i haven't try it yet because i'm using Win10 but i hope that will work!
