John C. Lang died in 1947 at the age of 67. He established the Jean Lang Dress Company in Minneapolis, MN in 1916. His obit noted that by 1947 the company employed “more than 100” people. I saw references to the Jean Lang Dress Company up to about 1970, but they may have been in business after that date.
I also saw hundreds of classified ads for sewers, machine operators, embroiderers, people to do alterations, etc. Based solely on that, I would assume they were a good-sized dress manufacturer and your lovely dress was just one of many, many styles they manufactured.