They cannot "hear," or reproduce those sounds. Many languages have things like this. The "clicking" of the !Iosa in southern Africa is a good example - most of us cannot make that sound. The Code Talkers in WWII were so successful because the Japanese could not "hear" the sounds made in Navajo. While they used codes, technically they weren't very good, but, the Navajo language communications were never broken by the Japanese.
It works the other way as well. "Peking," and "Beijing" are the exact same word. The capital of China is "京," which to English ears, sounds a bit like Peking, and Beijing. I live in the Klondike, which is an English form of a native word that also comes out as Trondek. Klondike, and Trondek are the exact same word - in the local native language. There are about 30 different ways of spelling it that have been used over the years.