Short answers, pulled from the story.
Korean is the native language of about 81 million people, mostly of Korean descent. It is the national language of both North Korea and South Korea, and it is also one of the two official languages of China's Yanbian Korean Autonomous Prefecture.
King Sejong the Great personally developed the alphabetic featural writing system known today as Hangul in the 15th century to promote literacy among common people. He introduced it in a document called the Hunminjeongeum, where it was called eonmun, meaning colloquial script.
In South Korea the language is called Hangugeo, with han drawn from Samhan and the Three Kingdoms of Korea, while in North Korea and China it is most often Joseonmal or Joseoneo, taken from Joseon. The names reflect different names used for Korea in the two states.
The United States Defense Language Institute places Korean in Category IV alongside Japanese, Mandarin and Cantonese Chinese, and Arabic, requiring 64 weeks of instruction compared with 26 weeks for Category I languages like Italian, French, and Spanish. The Foreign Service Institute also places Korean in Category IV, its highest level of difficulty.
Honorifics show respect toward the referent, the person being spoken about, while speech levels show respect toward the audience, the person being spoken to. Korean has seven speech levels, each with its own verb endings to indicate the formality of a situation.
The exact proportion is debated. Sohn estimated 50 to 60 percent in 2001 and 65 percent in 2006, while Jeong Jae-do estimated it could be as low as 30 percent, noting that colonial-era dictionaries included many unused Sino-Korean words.