Oh God. If you try to write in dialectic, God bless your soul. I'm from the southern US and still have to interpret half of what I hear local people say half the time.
There's a BIG difference between giving each character their own voice, and making the sentence's meaning clear. You shouldn't contract "it has" because it is not "it is". Clarity (for the most part) trumps authenticity.
Also, if one does include dialect, it's best to not overdo it and make your entire story utterly unreadable (case in point: the classic Ulysses, which is virtually unreadable, even by most literary experts).
Instead, use the full dialect for a paragraph or two, simply to establish how the person speaks, then ease off, relying on a few/handful of key phrases, which essentially remind readers that they're still speaking as they originally did.
If done properly, that usage keeps the character true to his background, while most readers won't even notice the transition from dialect to non-dialect. But then again, many disagree with my approach, so take the advice as you will. For me, I'd rather being able to understand a story than an author 'staying true' to the character for 387 pages!