How do I add another keyboards into Remix OS (Vietnam, Cambodia, Thailand Keyboards)

I have a problem which are I cannot add another keyboards namely, Vietnam, Cambodia and Thailand. I cannot change the Keyboard from English language to Vietnam, Cambodia and Thailand languages. I have tried Khmer Smart Keyboard, Cambodia Keyboard APK and Phum Keyboard for Cambodia keyboard; Keyboard ManMan, Gadgetdoor Thai Keyboard, Thai Gingerbread KB and Thai Pack Without ALM97774503 for Thailand keyboard. My host OS is Windows 7. I use VM-ware player to run Remix OS and Remix OS is marshmallow. How do I add another keyboards into Remix OS please? Kindly please help me out. Any suggestion, I would appreciate it.