Who, or "what" constituted a natural born citizen was well known to the framers. Jay would not have made such a suggestion to the others (Washington & the rest of those in attendance at the Constitutional Convention) unless there was a clear understanding of what that term meant. The definition comes from a source that not only were the framers familiar with, but the founders (many who were both) as well. And yes, even though most could not speak French, most read French (except, notably, Washington who would defer to Jefferson when such interpretation was needed).
NBC in the Constitutional drafts:
June 18th, 1787 - Alexander Hamilton suggests that the requirement be added, as: "No person shall be eligible to the office of President of the United States unless he be now a Citizen of one of the States, or hereafter be born a Citizen of the United States." Works of Alexander Hamilton (page 407).
July 25, 1787 (~5 weeks later) - John Jay writes a letter to General Washington (president of the Constitutional Convention): "Permit me to hint, whether it would be wise and seasonable to provide a strong check to the admission of Foreigners into the administration of our national Government; and to declare expressly that the Commander in Chief of the American army shall not be given to nor devolve on, any but a natural born Citizen." [the word born is underlined in Jay's letter which signifies the importance of allegiance from birth.] http://rs6.loc.gov/cgi-bin/query/r?ammem/hlaw:@field%28DOCID+@lit%28fr00379%29%29:
September 2nd, 1787 George Washington pens a letter to John Jay. The last line reads: "I thank you for the hints contained in your letter"
http://www.consource.org/index.asp?bid=582&fid=600&documentid=71483
September 4th, 1787 (~6 weeks after Jay's letter and just 2 days after Washington wrote back to Jay) - The "Natural Born Citizen" requirement is now found in their drafts. Madison's notes of the Convention
The proposal passed unanimously without debate.
Original French version of Vattel's Law of Nations:
Emer de Vattel, Le droit des gens, ou Principes de la loi naturelle, vol. 1 (of 2) [1758]
From Chapter XIX, 212 (page 248 of 592):
Title in French: "Des citoyens et naturels"
To English: "Citizens and natural"
French text (about citizens): "Les citoyens sont les membres de la societe civile : lies a cette societe par certains devoirs et soumis a son autorite, ils participent avec egalite a ses avantages."
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To English: "The citizens are the members of the civil society: linked to this society by certain duties and subject to its authority, they participate with equality has its advantages."
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French text (about "natural" born citizens): "Les naturels, ou indigenes, sont ceux qui sont nes dans le pays, de parens citoyens"
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To English, gives this: "the natural, or indigenous, are those born in the country, parents who are citizens"
Prior to the Constitution
"This 1758 work by Swiss legal philosopher Emmerich de Vattel is of special importance to scholars of constitutional history and law, for it was read by many of the Founders of the United States of America, and informed their understanding of the principles of law which became established in the Constitution of 1787. Chitty's notes and the appended commentaries by Edward D. Ingraham, used in lectures at William and Mary College, provide a valuable perspective on Vattel's exposition from the viewpoint of American jurists who had adapted those principles to the American legal experience."
Thomas Jefferson (for one example) had the 1758 version as well as a 1775 version in his own library:
Thomas Jefferson's Library: A Catalog with the Entries in His Own Order (under a section he titled "Ethics. Law of Nature and Nations."
In AUTOBIOGRAPHY by Thomas Jefferson, he states: "On the 1st of June 1779. I was appointed Governor of the Commonwealth and retired from the legislature. Being elected also one of the Visitors of Wm. & Mary college, a self-electing body, I effected, during my residence in Williamsburg that year, a change in the organization of that institution by abolishing the Grammar school, and the two professorships of Divinity & Oriental languages, and substituting a professorship of Law & Police, one of Anatomy Medicine and Chemistry, and one of Modern languages; and the charter confining us to six professorships, we added the law of Nature & Nations..." This was 8 years prior the the writing of the Constitution! [See the "Law of Nature & Nations" section of his personal library to get an idea of what he included in this curriculum in America's 1st law school].
Note: Vattel, is one of only 10 "footnotes" in Jefferson's Biography, from Yale.
After the Constitution
The same definition was referenced in the dicta of many early SCOTUS cases as well...some examples:
"THE VENUS, 12 U.S. (8 Cranch) 253, 289 (1814) (Marshall, C.J. concurring) (cites Vattel’s definition of Natural Born Citizen)
SHANKS V. DUPONT, 28 U.S. 242, 245 (1830) (same definition without citing Vattel)
MINOR V. HAPPERSETT, 88 U.S.162,167-168 ( 1875) (same definition without citing Vattel)
EX PARTE REYNOLDS, 1879, 5 Dill., 394, 402 (same definition and cites Vattel)
UNITED STATES V WARD, 42 F.320 (C.C.S.D. Cal. 1890) (same definition and cites Vattel.)"
http://www.scribd.com/doc/17519578/Kerchner-v-Obama-Congress-DOC-34-Plaintiffs-Brief-Opposing-Defendants-Motion-to-Dismiss
A detailed, historical, etymology of the term "Natural Born Citizen" can be found here: http://www.greschak.com/essays/natborn/index.htm
Prior to Jay's famous letter to those in attendance at the Constitutional Convention, we see (one of many exchanges between the founders) a letter from Madison ("father" of the Constitution) to Jay:
"James Madison, as a member of the Continental Congress in 1780, drafted the instructions sent to John Jay, for negotiating a treaty with Spain, which quotes at length from The Law of Nations. Jay complained that this letter, which was probably read by the Spanish government, was not in code, and "Vattel's Law of Nations, which I found quoted in a letter from Congress, is prohibited here.[29]"
From: Life, Liberty, and The Pursuit of Happiness. How the Natural Law concept of G.W. Leibniz Inspired America's Founding Fathers.
Vattel's Law of Nations, built upon "natural law - which has it's roots in ancient Greece, was influenced by Leibniz.
Even Blackstone affirmed the basis of natural law:
"This law of nature, being co-eval with mankind and dictated by God himself, is of course superior in obligation to any other. It is binding over all the globe, in all countries, and at all times: no human laws are of any validity, if contrary to this; and such of them as are valid derive all their force, and all their authority, mediately or immediately, from this original” (1979, 41). In this passage, Blackstone articulates the two claims that constitute the theoretical core of conceptual naturalism: 1) there can be no legally valid standards that conflict with the natural law; and 2) all valid laws derive what force and authority they have from the natural law."
Founder and Historian David Ramsay Defines a Natural Born Citizen in 1789.
David Ramsay (April 2, 1749 to May 8, 1815) was an American physician, patriot, and historian from South Carolina and a delegate from that state to the Continental Congress in 1782-1783 and 1785-1786. He was the Acting President of the United States in Congress Assembled. He was one of the American Revolution’s first major historians. A contemporary of Washington, Ramsay writes with the knowledge and insights one acquires only by being personally involved in the events of the Founding period.
Ramsay REAFFIRMS the definition a Natural Born Citizen (born in country, to citizen parents (plural)) in 1789 A Dissertation on the Manners of Acquiring the Character and Privileges of a Citizen (1789)
The Naturalization Act of 1790, which states (in relevant part) "that the children of citizens [plural] of the United States that might be born beyond the sea, or out of the limits of the United States, should be considered as natural-born citizens"
Of course, the Act of 1790 was repealed by the Act of 1795 (which did NOT attempt to define or extend the definition for NBC). What the 1st Congress had tried to do in 1790 was to EXTEND the known definition (of born in country to citizen parentS) to those born outside of sovereign territory, to citizen parentS. Of course, they can't do that. Congress (by itself) doesn't have the Constitutional authority to define (or EXTEND) the term "Natural Born Citizen." Only a SCOTUS decision on the intent of the framers, or an amendment to the Constitution can do that.
It's interesting to note that (non binding) Senate Resolution 511, which attempted to proclaim that Sen. John McCain was a "Natural Born Citizen" because he was born to citizen parentS, even they referenced the (repealed) Naturalization Act of 1790: "Whereas such limitations would be inconsistent with the purpose and intent of the `natural born Citizen' clause of the Constitution of the United States, as evidenced by the First Congress's own statute defining the term `natural born Citizen'". Obama, himself, was a signatory of that resolution knowing full well (no doubt) the requirement has always been about 2 citizen parents.
John Bingham, "father" of the 14th Amendment, the abolitionist congressman from Ohio who prosecuted Lincoln's assassins, REAFFIRMED the definition known to the framers by saying this:
commenting on Section 1992 said it means “every human being born within the jurisdiction of the United States of parents not owing allegiance to any foreign sovereignty is, in the language of your Constitution itself, a natural born citizen.” (Cong. Globe, 39th, 1st Sess., 1291 (1866))"
The point is, with the exception of the repealed Act of 1790 which tried to EXTEND the definition, the meaning of the term "Natural Born Citizen" has ALWAYS been about being born within the sovereign territory (& thus jurisdiction) of the U.S. to 2 citizen parents (& therefore parents who do NOT owe allegiance to another, foreign, country).