There's always some people who are gifted at becoming fluent in a new language. For others it takes longer to become as good. These are the idols that they say are natural at Korean:
Yuqi - many people just assume she's Korean
Sana - obviously they could tell she's a foreigner but her Korean is almost native level
Kyulkyung* - when she was active with IOI Koreans commented her Korean was very good
Sorn - never seen any Korean complain about her language skill
Minnie - similar to Yuqi, she seems gifted in terms of language acquisition
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These are the idols who are still improving their Korean, although they are pretty good now:
Shuhua - it's well known she had difficulties with Korean but they say she's much better now
Mina - people can tell she's not a natvie Korean speaker
Lisa - similar to Mina
Tzuyu - i'd put her between Mina and Shuhua, her Korean improved over time
Momo - Koreans made fun of her accent, especially her former "boyfriend" Heechul during the Cheer Up days
Soojin - she was born in korea but IDLE members says she's the "hidden foreign member" because she flubs her Korean
I didn't include the WJSN China line because they seem to have just started learning Korean but then had to leave quickly back to China. There's also other foreign idols who are between these two groups.
wow yuqi's an ace at everything and the korean language is 
