@burcinc-- I'm not an expert on citrus fruits. As far as I know, mandarins, clementines and tangerines are similar but they are not the same. Their flavors and appearance vary a little bit.
I think the closest citrus fruit to a naartjie is the satsuma orange or mandarin which is Japanese. Some believe that the naartjie is of Japanese origin and was taken to China from Japan. This makes sense because naartjie exists in both countries and is known as migan or mikan.
An expert can explain the differences between these fruits in detail. But Americans who want to eat naartjie can buy satsuma instead, which is available across the US.