Hey friend!

It is absolutely not necessary to have Computer Science as your subject in 11th and 12th to take CD in iit's . So you should completely Focus on jee preparation . Don't worry about such things . People take CS to have an advantage in CS in the college . But don't worry . Everything will be covered in college subjects . You can easily a learn a particular language like c++ , c after giving your jee main when you are 2-3 months free .

All the best friend!