According to the DVD commentary, the Japanese man who repeats the "incredibly large penis" trick is based on someone Trey Parker and Matt Stone met in Beijing.
All of the Japanese, with the possible exception of "Garrison-san", is correct Japanese. Trey Parker studied Japanese in college and speaks it fluently.
This is the last episode where Mary Kay Bergman voices Wendy Testaburger before her death on November 11, 1999.
The Japanese term chinpoko means "small penis". The entire phrase "Chinpokomon" means "penis [pocket] monster" in Japanese. The word Poké is short for pocket in Japanese, which is where the word "poko" comes from.
Midway one of the kids states that Kenny has had a seizure. This is a reference to Dennô Senshi Porygon (1997), which was never aired outside of Japan, as the flashing light patterns had induced seizures in many children.