In-movie explanation: 1
Bane was reportedly born and raised in the Pit, a hellish prison
located within a Middle Eastern country. He came to regard the prison
as a home, a place where he learned "the truth about despair."
From Bane himself:
"It's based on a guy named Bartley Gorman," he told us. (So, not Mr. Belevedere?) "He's the king of the gypsies, and he's a boxer, a bare-knuckle boxer, an Irish traveler, a gypsy." 2
According to Tom Hardy, he based his voice for Bane on Bartley Gorman
(1944 - 2002), an Irish Traveller who was the undefeated Bare-knuckle
boxing champion of the United Kingdom: "The choice of the accent is
actually a man called Bartley Gorman, who was a bare knuckle fighter,
a Romani gypsy. So I wanted to underpin the Latin, but a Romani Latin
opposed to Latino." 3
The Mehrangarh fort is just a location they used for shooting the movie. If we still consider this to be the Pit, then we can assume Bane picked up the Indian dialect since he was born and raised in the Pit which results in his accent...