Does the name Scientific American consist of two adjectives? What is the substantive?
Scientific is the adjective form of science, and American is a noun meaning a citizen of the United States. The intended audience of the original Scientific American, first published in the U.S. in 1845, were people with either a professional or avocational interest in the many fields of science. If you fancied yourself a person of science, you were the target audience. The magazine today still boasts, "A third of Scientific American readers hold postgraduate degrees."
Also, the magazine was actively involved in the early patent process in the U.S., helping scientists and inventors protect their inventions and disseminate their ideas. According to the magazine's website:
In an era of rapid innovation, Scientific American founded the first branch of the U.S. Patent Agency, in 1850, to provide technical help and legal advice to inventors. A Washington, D.C., branch was added in 1859. By 1900 more than 100,000 inventions had been patented thanks to Scientific American... For a century, Munn & Company retained ownership of the magazine, which chronicled the major discoveries and inventions of the Industrial Revolution, including the Bessemer steel converter, the telephone and the incandescent lightbulb. Edison presented the prototype of the phonograph for inspection by the editors, and Samuel Morse, father of the telegraph, and Elias Howe, inventor of the sewing machine, were frequent visitors to the offices in downtown New York City.
There is not clear-cut a line in English between nouns and adjectives. Nouns can generally be used attributively. Adjectives can often be used substantively.
"American" used attributively means "from America", and "American" used substantively means "(a person) from America". Semantically the difference in the case of the magazine is negligible.
Though only the founders or publishers of the magazine can say for sure, in my judgment the name simply means "(a magazine for) American persons who are scientific."
MW defines the noun American as
1: an American Indian of North America or South America
2: a native or inhabitant of North America or South America
3: a citizen of the United States
I think the Russian equivalent of this title would be Научний Американец (non-linguist's transliteration--Nauchniy Amyerikanyets). Though I agree that a journal should not be required to have a noun, I also agree that a title that translated to Научний Американский would annoy me as well. Please forgive me if my Russian spelling is off. It has been awhile!