So when you go to represent the speech of a character with a regionally or ethnically distinct speaking pattern, you keep it as simple as you can. You might have your character use a few distinctive vocabulary words, and you might have him pronounce a few things differently, as captured in what you write. Whatever it is, two rules:
1. Keep it simple.
2. Keep it consistent.
Think of it as a little algorithm or a translator. You put normal speech in one end, apply the couple of rules for the character, and his distinctive speech comes out the other end.
Here's an example from what I'm writing now. Bob speaks with some sort of unidentified Estuary English (Hackney / Essex / somefink), and I write his speech with a few simples rules, which you can see in action here.
* * *
Though Charlie was a good boy and an obedient son, he felt a small twinge of envy at seeing two boys so dirty—he’d never been allowed to be that filthy in his life.
“You can stop hit wiff the mister right there an’ call me Bob,” said the other, while Mr. Pondicherry examined the calling card politely. “An’ hif you’ve got to hextinguish me from other Bobs, I’m ’Eaven Bound.”
“Distinguish, Bob,” Chattelsworthy hissed.
“Yeah,” Heaven Bound Bob agreed. "I fought that's what I said."