Q. I can't see alphabet in my language, why?

It is because your system doesn't have the right font.

Q. Is there anything can be done to make my system display my language?

Yes. There are several ways:
1. If your device is a Samsung, try this font pack, it may have your font.
2. Install a custom ROM that supports your language.
3. Get your Android rooted and install your font to it with FontChanger.
4. Install MyAlpha app

Option 1.

Only works on Samsung Galaxy Series. Try it

Option 2.

Please google for: custom ROM android

Option 3.

Please google for: root Android
Once your system get rooted, install FontChanger

Option 4.

Choose this only if the options above are too complicated for you.
Install MyAlpha app. You can see and type your language inside this app but other apps will remain showing boxes or nothing for your language. After finish editing, you can either copy and paste or share the text via email, SMS, twitter... (again, you see boxes or nothing in other apps, but your recipient will see them if their systems support it)

Q. I can see my language now, but some of them don't render correctly.

It is because the font rendering engine of Android system isn't prepared to handle complex font rendering. Again, you can certainly try to look for a custom ROM that supports your language.

Other useful sites:

CyanogenMod, Khmer Android