An interpreter is a bilingual person who can facilitate communication with her bilingual and bicultural competency. It is not easy to be completely bilingual and bicultural, and you could even grade interpreters in the scale of 0 to 100. For example, an ideal Korean interpreter would be someone born in Korea, spoke Korean language as mother tongue, and learned English as second language either in the U.S. or from a native teacher in Korea before 12 years to speak English without accent.
Then she went to a college in Korea, to have a high level of Korean language. Then she went to a graduate school in the U.S. or worked in the U.S. to further develop her English.
Another ideal type Korean interpreter would be someone born in the U.S., spoke English as mother tongue, and learned Korean as second language either in Korea or from a native Korean teacher in the U.S. before 12 years to speak Korean without accent. Then he went to a college in the U.S. and went to a graduate school in Korea or worked in Korea to further develop her Korean.
However, such ideal types rarely exist. There could be one or two such interpreters in the entire world. Naturally, Korean interpreters have varying levels of competency in English and Korean. Those who grew up in Korea and went to a college in Korea would have a high level of Korean competency and probably accuracy, but invariably have accents.
Those who grew up in the U.S. might not understand difficult Korean concepts especially when they are made up of Chinese characters.