https://en.wikipedia.org/wiki/North%E2%80%93South_differences_in_the_Korean_language

The Korean language has diverged between North and South Korea due to the length of time that the two states have been separated. Underlying dialectical differences have been extended—in part by government policies, and in part by the isolation of North Korea from the outside world. There are some differences in orthography and pronunciation, and substantial differences in newer vocabulary; whereas the South tends to use loanwords from English, the North tends to use loanwords from Russian or construct compound words.

Problems

During the 2018 Winter Olympics, the two Korean countries decided to play jointly for the Korea women's national ice hockey team. This led to issues with the South Korean athletes communicating with the North Korean athletes since the former uses English-influenced words in their postwar vocabulary, especially for hockey, while the latter uses only Korean-inspired words for their postwar vocabulary.[6]

The language differences also pose challenges for researchers and for the tens of thousands of people who have defected from the North to the South since the Korean War. The defectors face difficulty because they lack vocabulary, use differing accents, or have not culturally assimilated yet so may not understand jokes or references to pop culture and K-pop.[6]