At IIT Madras, you will find students coming from all across the country. These include states like Gujarat, Karnataka, Assam, Kerala, Orissa, West Bengal, Haryana, and Punjab. Despite this, the two most frequent languages are English and Hindi.
Another majorly spoken language is Telugu. You should not be really surprised because more than 50% of the students at IIT Madras are from Andhra Pradesh and Telangana. Despite the institute being located in Chennai, not many students speak Tamil.
There are quite a few North Indian students as well. Other than this, only 10% of the students speak Tamil.
Overall, while there might be a larger number of students from the South, you will also have a healthy mix of people from other states.