@Alljut: In CSS there is only one paper in islamiat so you can opt it either in English or Urdu. Thats upto you.
Regarding the choice of Language, I have heard that attempting it in English gives a bit extra edge otherwise the rest is the same. I personally attempted it in Urdu and scored 70 while studying just one book i.e. by Laqa Ul Hassan Zahid of Dogar Brothers. But there are students who suggest other books as well. I believe follow this one book first and then if you had time, go for any other good book as a supplementary material.

All the best.