To write the script using verbatum text was a miss. Books written in that time period did not reflect the actual spoken language. At the turn of the century (that last one for you young whipper-snappers), it was poor form to use contractions in written text (e.g., shouldn't, couldn't, don't...). But nobody in this part of the country actually talked in the formal, proper English.
It wasn't supposed to be an accurate representation of period speech. It was supposed to be a film version of the book.
At first I thought the language was kind of stilted, but it didn't bother me after a little bit. Also I think that the comedic timing of some lines ("I do not know him" after they cut the guy down from the tree) worked better that way.
Also the book was written in 1968, and I think they'd gotten over contractions by then.
Matt Damon was a miss. He was too soft to play the role of weathered Texas Ranger.
I haven't read the book, but I got the impression that LaBoeuf was supposed to be a cocky jerkass, not a seasoned Texas Ranger. Damon plays that part well.