$ sudo apt-get update
$ sudo apt-get install ibus-libthai 

ibus-libthai is installed in your system.

Make ensure the ibus-libthai package were installed using the commands given below,

$ sudo dpkg-query -l | grep ibus-libthai *

You will get with ibus-libthai package name, version, architecture and description in a table

ibus-libthai package basic information: