I'm Korean, but I speak Chinese very standard. I didn't recognize that I was Korean. Speaking Korean is just like foreigners speaking Chinese.
Actually it has a lot to do with the environment! Just like some people in China grew up in the United States and came into contact with English. Of course, English is very fluent.
Koreans speak Korean, not Korean. The two languages are different, but there are still differences!
Korean, like "Zha" and "Gahaya" as we say in the northeast, will be laughed at in the south; Korean is equivalent to standard mandarin, and speaking Korean to Koreans is also the same, which will make Koreans feel very rustic.
Moreover, there are many foreign words in Korean, and Koreans can understand a lot less.
In fact, the biggest difference between Korean and Korean lies in grammar.
Well, that's all I know.