I have not come across a S Indian who knew Hindi & did not reply / talk in hindi when spoke to in the same language.
The South too has four prominent languages -Tamil, Telugu, Kannada & Malayalam.
So nothing much .
Actually its quite like Urdu being the national language of Pak but not spoken or the preferred language outside Punjab.
I have come across many South Indians who would reply in English, even when asked in simple Urdu or Hindi. Many of my Indian friends have confirmed this fact to me.
Urdu is widely understood and used all over Pakistan. It is only in some very small far flung areas in Sindh where it is not yet understood.
Go anywhere else and you'd not be responded to in Urdu.
This is not the case of Hindi language use in India though.
Hindi is not national language,its majority speaking language.
Which language is India's national language.