I use KDE Desktop environment for long time.
The recent KDE5 is good, sleek and beautiful.
To type in Tamil, I use ibus.
Installed it and configured Tamil99 keyboard layouts.
sudo apt-get install ibus-m17n ibus m17n-db m17n-contribibus-gtk ibus-qt4
This links helped to setup ibus.
Still in KDE5 , ibus is not working only in KDE applications. But, it works well with GTK based applications like Firefox.
In order to resolve this problem add
to ~/.xprofile and restart your X user session. ie. logout and login.
After doing this, I can type in tamil using ibus in all KDE applications.