In the 80s there was a group of Russian girls (almost identical, sisters or cousins) that spoke to each other in a strange language. They didn't responded if so

In the 80s there was a group of Russian girls (almost identical, sisters or cousins) that spoke to each other in a strange language. They didn't responded if someone spoke to them in English. It turned out they spoke Russian, but the words and pronunciation young kids use at home is very different to what they teach as Russian in America from textbooks and language tapes. It took a native bilingual Russian speaking doctor to figure out that mystery.