They are speaking Japanese and English in this video because of the Japanese and American tourists standing in front of them. They also perform in Mandarin, Korean, and a few of the guys know French and German.
There are several places where you can watch this traditional, royal snack being made in Seoul. This was filmed in Insadong.
Ggultarae is fermented, 2 week old honey and malt, stretched by hand over and over in corn starch to create 16,000 strands of hair-like strings. Then it is rolled up with various fillings of your choice, such as peanuts or chocolate.
This snack was once only eaten by the royal family, but has since become an available snack for anyone. Originally, this snack was introduced to Korean royalty from China.