What nationalities are mainly in Chennai (Madras)?
Someone already mentioned all Indians, but just wanted to add that unlike the good ole USA where people find hard to learn a 2nd language, India has tens of languages and people usually know 2-3 of them. See this wikipedia link
http://en.wikipedia.org/wiki/Languages_of_India
And the fun part is, many of the languages here are spoken in other countries too, and hence the notation in wikipedia giving the numbers only for India. Examples ... Bengali, Punjabi, Sindhi, Tamil, Urdu and Maithili have probably more speakers in other countries while all of them having 10+ million speakers just in India.
Oh, did I forget English !! I think we are shooting to be the #1 english speaking country, if not already there.