Good luck. What I can tell you is that in 2008 the US ratified the Hague Convention on International Adoption, which was written in 1993. According to at least one adoption-related source, this was the first such treaty. And it says nothing about citizenship, only that the receiving nation must have determined that the child " is or will be authorised to enter and reside permanently in that State."
See Perkins v Elg.
“Miss Elg was born in Brooklyn, New York, on October 2, 1907. Her parents, who were natives of Sweden, emigrated to the United States sometime prior to 1906, and her father was naturalized here in that year. In 1911, her mother took her to Sweden, where she continued to reside until September 7, 1929. Her father went to Sweden in 1922, and has not since returned to the United States. In November, 1934, he made a statement before an American consul in Sweden that he had voluntarily expatriated himself for the reason that he did not desire to retain the status of an American citizen and wished to preserve his allegiance to Sweden.
In 1928, shortly before Miss Elg became twenty-one years of age, she inquired an American consul in Sweden about returning to the United States and was informed that, if she returned after attaining majority, she should seek an American passport. In 1929, within eight months after attaining majority, she obtained an American passport which was issued on the instructions of the Secretary of State. She then returned to the United States, was admitted as a citizen and has resided in this country ever since.”
http://www.freerepublic.com/focus/f-news/2499682/posts
http://supreme.justia.com/us/307/325/case.html#328
In the decision, they also cite this:
“Under the treaty, and in harmony with the American doctrine, it is clear that Steinkauler, the father, abandoned his naturalization in America and became a German subject (his son being yet a minor) and that, by virtue of German laws, the son acquired German nationality. It is equally clear that the son, by birth, has American nationality, and hence he has two nationalities, one natural, the other acquired. . . . There is no law of the United States under which his father or any other person can deprive him of his birthright.”