North Korean vs. South Korean
Due to fifty years of separation, differences have arisen between the North Korean and South Korean languages. When Noh Moo Hyun visited Pyongyang in October 2007, it was said that Kim Jong Il could only understand about 70% of what was said, due to these differences. While grammar, for the most part, remains the same, vocabulary and pronunciation have changed quite a bit.
Most notably, South Korea tends to use English words for neologisms. North Korea either invents a new word from scratch or, in some cases, it imported the word from Russian. An example of this is the word “tractor.” South Korea uses the word 트랙터, derived from English while the North uses 뜨락또르 derived from Russian. For a comprehensive vocabulary list of North-South differences, check out this site (in Korean)