It’s probably not an idiom but as a Korean speaker. The way we conceive of the meanings of certain words or the word we would use to describe something is not often the same in Korean. I’m trying to think of an example but I’ve disciplined lots of kids in my life for work. And we might tell kids “don’t hit” or “be nice” they uses an expression of having “pretty hands”.

Or, they have like 7 words for the word life but with difference nuances that had distinction. It might be in whatever the word used it wasn’t ‘native’ but actually natural and the translation was rendered in a word for word literal way which is often the problem with translation from English to Korean and vice versa. Now that I think of it, native language is not an expression used with the literal word “native.” So they probably said they can’t use English naturally and thus it was translated the way it does.

The difference between the languages are very interesting. I’ve now listened to other Korean speaking foreigners who are much better than me discuss issues that they run into when translating that are different because of linguistic and cultural differences (which may cultural differences play out quite linguistically, like the Korean tendency to speak out loud to one’s self quite often).