Nowadays, the Khmer Language is supported most of the Linux distribution. So it is not difficult to install or adding the keyboard layout to the platform. I mean that after you installed the OS to your machine and just add the language and the keyboard layout.
In this tutorial, I will show you step by step the pictures that I screenshot. Let's Started.
This screenshot shows you the ubuntu-desktop versions 20.04 LTS. You can identify by the take screenshot (Focal Fossa). Anyways, It doesn't differ much from another version.
[screenshot 1]1. Click it (It will show you the search box with the recently searched)
2. Click in the box and type "language . .". You do not complete all words. just type language and then it will autocomplete for you then click on it.
3. Click on the "Language Support" icon.
[screenshot 2]
After "Language Support" was lunched please click on:
1. Language Tab. (If you see the Khmer, Central there you can move to [screenshot 6] without installing the language.
2. Click on Install/Remove Language...
[screenshot 3]
4. Scroll until to see your language (Khmer, Central). and tick it to install.
5. Click Apply button
[screenshot 4]
Screenshot 5 is talking about adding keyboard layout.
1. Click on the 9 dots icon.
2. Type the word "setting". again you do not type a complete word.
3. Click on the Gear icon. 
[screenshot 5]
1. Scroll down and click on the world icon with the word Region and Language
2. Click on + sign
3. Click on 3 dots [...] for more Language.
4. Type the word "khm" or "Khmer"
[screenshot 7]
5. Click on Other to see our search
7. Click on Apply (the green button) to add keyboard language.
[screenshot 8]
[screenshot 9]
[screenshot 10]
After installed completed you will see the language on the top bar and you click to change it or just pres keyboard to change it.
Finally, I hope this tutorial can help you, if so. Please click like and share this tutorial with other people to see it too.