Korean is the official language of North Korea. It has slight variations to the Korean spoken in the South due to the isolation from a lot of the outside world after the end of the Korean War.
If you have learnt Korean in South Korea, you will be able to have conversations with absolutely no problem. The main difference appears with the level of formality and some vocabulary words. North Korean is much more formal than in the south. In fact, the way the people in South Korea speak can sound quite rude in some contexts in North Korea, so if you are a Korean speaker then use the politest version of grammar you can think of.