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Yes, defectors exist, but they're so vanishingly few that it makes no sense to ask. Average defectors are something like 1-2k/year, and by comparison, about 16-17k S. Koreans immigrate to the US every year. So the chance that the person you're talking to is from N. Korea is incredibly small, not only do very few people actually escape N. Korea, they likely largely stay in S. Korea, where they speak the language.

"Korea" is the proper name for both countries, the "North" and "South" descriptor comes from other countries to disambiguate them. The official name for S. Korea is "Republic of Korea," and the official name for N. Korea is "Democratic People's Republic of Korea." So if you're wanting people to fully state the country they're from, you'd probably be more confused because both countries' official names include "Republic of Korea." But you'll likely never talk to someone from N. Korea, so the whole question is moot. If they say they're from "Korea," 99.9% of the time they'll be from South Korea, and if you live outside of S. Korea, it's probably like 99.999%.