There is a messages_kh.properties in Core (here) meant to be for the Khmer language.
But unfortunately the locale ID is erroneous, Khmer would be ‘km_KH’ or at the very least ‘km’ (but not ‘kh’).
(Q.) How do we correct this, is it just a matter of renaming the file to messages_km.properties through a PR or there are other things to do in regards to Transifex… etc?
openmrs-core is not translated via transifex, so I think you can just rename the file. (If you’re using the Khmer locale in some implementation you’ll probably be the first to identify any errors that pop up.)