The earliest instance of "I seen you" that a Google Books search turns up is from a 1733 edition of a 1620 translation by "Mr. Shelton and Mr. Blunt" of Cervantes's Don Quixote, which they title The History of the Valorous and Witty Knight-Errant Don Quixote of the Mancha. The wording arises in a speech by Dorotea, whom Wikipedia describes as "a modest young woman, whom Ferdinand promises to marry and then leaves." Here is the relevant passage:
I am she, which sometime immured within the Limits of Honesty, did lead a most contented Life, until it opened the Gates of her Recollection and Weariness, though to thine Importunity, and seeming just and amorous Requests, and render'd up the keys of her Liberty, a Grief by thee so ill recompenced, as the finding myself in so remote a Place as this, wherein you have met with me, and I seen you, may clearly testify; ...
Clearly "I seen you" in this instance is a short form of "I have seen you," with the have understood from the earlier wording "you have met with me."
The first Google Books match for "I seen" in the sense of "I saw" is much later. One possible match is from The Life of John Metcalf, Commonly Called Blind Jack of of Knaresborough (1795):
Metcalf took it for granted that his companion had seen one of these [will-o-the-wisps] but for good reasons declined asking him whereabout the light was; and to divert his attention from this object, asked him, "Do you not see two lights; one to the right the other to the left?" "No," replied the gentleman; "I seen but one light, that there on the right."—"Well then, Sir," said Metcalf, "that is Harrogate."
This episode, which the author says took place in 1735, involves the speech not of an illiterate workingman or a criminal, but of an English "gentleman" who happens to be relying (unwittingly) on a blind man to serve as his guide from the city of York to Harrogate—a distance of approximately 22 miles). It is certainly possible that this instance is simply a typographical error (seen for see), since the gentleman being quoted may be looking directly at the light as he speaks. Nevertheless, someone at the printer's shop put the n there, and presumably someone read it and thought it looked okay.
The first unmistakably intentional use of "I seen" in a Google Books search result is from "Tim Bobbin," Plebeian Politics, or The Principles and Practices of Certain Mole-Eyed Maniacs Vulgarly Called Warrites (1801), a dialogue written in Lancashire dialect:
Tum [Grunt]. Zuns, mon! boh I seen th' dey when won wur likker t' ha' bin breant wi' thoose foos for seyink hawve oz mitch oz so; heaw did e kum off wi' em?
From Christian Johnstone, Clan-Albin: A National Tale (1815), the nation being Ireland:
"...But, after all that, I must, in the devil's name, be talking and joking to make 'em laugh, and acting Brian Baru and the like, as I seen in the treater; so I was packed off for my cleverness, and Ellis the Englishman taken, who tells no lies, nor much truth neither, as he seldom says any thing good or bad."
Next comes a double instance in trial testimony in Trial of Frederick Eberle and Others, at a Nisi Prius Court, Held at Philadelphia, July 1816 (1817). The speaker in the recorded testimony is Henry Schrader (identified at the beginning of his testimony as "Has been at the meeting of the German society, does not belong to it"):
Vanderslice was going to take the man who had took the book from Mr. Witman; I heard Geyer call to Vanderslice, "catch that man"—they went all together in a lump; I could not tell who they had or who it was, I seen they had hold of a man by the coat—I did not know anything further—I went over to the school house on Fourth street and stood there a little bit[.]
...After a while, I cannot recollect the time, whether it was before dinner or after, I saw a crowd of people on the pavement near the church; Vanderslice was among them, and I seen Vanderslice had hold of a man there, and some of them wanted him to leave the man go; one of them, Spiess clapped his hand upon his shoulder, and said "Vanderslice, you had better leave that man alone."
So as of 1817, we have definite sightings of the wording "I seen" situated in Lancaster, England, among fictional local rustics; in Ireland as spoken by a fictional Irishman; and in Philadelphia, Pennsylvania, in actual trial testimony given by a German American. All of these instances precede any recorded instance from the U.S. South.
I suspect that the use of "I seen" for "I saw" has been reinvented many times in the history of English, particularly since the question "Have you seen X?" invites the untutored response, "Yes, I seen X"—or for that matter, the correct response, "Yes, I've seen X" voiced in a way that sounds to the untutored listener very much like "Yes, I seen X."
Update (June 20, 2023): A seventeenth-century instance of 'I seen you'
A search of Early English Books Online turned up the following instance of "I seen you" (in the sense of "I saw [that] you") from a pamphlet published in 1681. It appears in a letter, dated October 21, 1681, from John Fitz-Gerrald (described by the hostile author of the pamphlet as "an Irish man and formerly a Romish Priest" who had been imprisoned for debt in the Marshalsea, where the letter was written), reprinted in "A Discovery of One Sham More: Design'd Against Three of His Majesties Justices of the Peace for the County of Surrey" (1681):
Mr. Parcurst,
Sir, I was In great hopes of my Libertie, when I seen you had A hand in It, But I begin to despaire of it by reason of your delays, And I am Certaine if you had stured In it, It had been done since you were heare last, but happily you will say you durst not meddle In it But Alas? ...
This instance is notable in using "I seen" not as a short form of "I've seen" but as an alternative form of "I saw." It is also significantly older than any similar use of "I seen" that I have found and is less ambiguous than the instance from 1795 cited in my original answer. It also bears noting that the framing here isn't simply "when I seen you" but "when I seen [that] you had a Hand in it..."
