Suimasen is actually Japanese. If you look at the top left of the video there is actually a definition of the word in the video. This is pretty common with Japanese videos, you need to be aware of these definitions popping up in available space on the screen.
The reason for this is that there are a lot of words in Japanese where we don't have a similar word or concept in English, so to try and keep the original meaning they will use the Japanese word and try to explain it.
English then adopts some of these, oniichan and sempai are 2 that come to mind.