I hope you realize there is a big difference between the North Korean population and the S. Korean population. In North Korea, they are cut off from the outside world. The North Koreans are Communist. I would say, probably, for the most part, the North Koreans don’t speak foreign languages, but if they do, it would be Chinese.
You see, the reason they are Communist is because the Chinese made them Communist. When America helped Korea kick Japan out of Korea, there were two ways of thinking in Korea. One was that they wanted a Democracy. They already had a Korean government set up in secret outside of Korea that was Democratic. They had demonstrations downtown Seoul campaigning for Democracy and big Democracy meetings. However, there was a portion of them influenced by China. The Chinese were Communists, and they wanted to take Korea over just like that took over Tibet and other places like it. They were already in North Korea. One man told me that during both the war with the Japanese and the Korean war, there were Chinese in Korea, and some of the soldiers were occupying his house. The Korean war came about because there was a split in philosophy because of the Chinese pushing on the Koreans. They split the country in two, and the Northern part became Communist like China. that is why I say if any of the North Koreans speak a foreign language, it would be Chinese. However, the N. Koreans are very cut off from the outside world, so I doubt many at all speak foreign languages.
As for South Korea, English is the preferred foreign language. Some of them also speak Japanese because Japanese is easier for them to learn to speak and some of them learned it when they were under Japanese domination. However, S. Korea is very open to the rest of the world. They have a saying, “This is a global world.” They know that for their economy to thrive, they need to trade with other countries. They have deduced that English is the best language for communicating with most of the world’s population, and they are right. You can go to any country in the world and find someone who speaks English. That is why Koreans learn to speak English. They want to do business with other countries. They want to travel and see the world. Some of them want to become missionaries. They want to study abroad, They all think English is the key to doing all those things.
There are English schools set up all across S. Korea. Teaching English is a big business in S. Korea. Right now, S. Korean native speakers of English teachers are being sought after big time in S. Korea, even during the world’s pandemic of the coronavirus. If you go to Facebook, you will find a page there where numerous recruiters of English teachers are advertising wanting people to come to Korea to teach English. The Koreans don’t expect anyone to learn Korean, and they are all trying to learn English. To get a good job in S. Korea, they have a test called the TOEIC. a government English test. Every young person studies to take that test and get a decent score if they want a decent job in S. Korea.
As you can see, there is a whole different mindset between N. Korea and S. Korea, and it influences what language they study.