Not telling nobody

I surfed past Spiderman 2 Sunday; it was on the scene where the people bring Spidey inside the train. Two boys bring him his mask, and one says, "We'll not tell nobody."

That sounds downright weird to me, a mix of registers (or styles). "We'll not" sounds extremely formal, compared with "we won't". And then the second negative... It just sounds weird. Either "We'll not tell anyone"or "We won't tell nobody" would be more natural.

But that might just be me.