Actually, they are :
41% Saoudis
19% Lybians
8% Syrians
But, someone like Tarkhan Batirashvili, a Georgian, is actually known as Abou Omar al-Shishani, one of the most important leader of ISIS. Many Tunisians leaders have been recently executed.
New ISIS fighters now come (via Turkey) from China, Armenia, Georgia, Kazakhstan.
It's not so difficult to change their battlefield. A lot of them came from Libya. And they could easily move toward central Asia.