Here's how I did
(1)Copy DroidSanThai.ttf from other device to your phone.
(2)Download iFont from Google play.
(3)Click above DroidSanThai.ttf and it will triger iFont to do the rest of the work.

there's no need to reboot your phone. Thai will appear again in most of your app including your Thai keyboard. But the built-in email still does not support Thai.

I could not see Thai characters on my phone after I did the update of 4.2.2 on Samsung S4 as well.

I used iFonts app from PlayStore and you need to restart the your phone after you select a custom font so to stop it from using Default font. I used a font called Slices which seem to be able to enable the Thai fonts on the S4. The following thread explains it go to page 4.

Fonts for AT&T Galaxy S4 ? - Page 6 - xda-developers


Don’t revert to Default Font otherwise it will go back to blank again and you will need to use iFonts app again to change it and restart your phone to come into effect.


Note Thai Character doesn't appear on the Samsung Keyboard as they did prior to 4.2.2 update but I used TSwipe app which works fine.

My model is GT-I9505

I bought the phone online and its seem to be international model. It worked fine before but a day ago it asked me to update so I did.
and now I cannot see any Thai on my phone. It shows as blank. Even though I added Thai keyboard, i still cannot see anything.
I tried to restore but it still did not work.
Please help. Thank you