Please read this, This is ypu understand the korean - japanese thing :)

I want to help yall understand whats happening because I remember there was alot of confusion when Chapter 21 came out. I recommend reading the novel because it will help you understand everything better. but if you dont want to read the novel heres what happened in it, since it more detailed on writing :)
Remember what I will be writng what happened in the NOVEL (its more detailed), Here in the manga they tend to rush things which leads to making many people confused as to what is happened.

So in the manga, Minjoon is Korean whole everyone else in the story is Japanese. Minjoon is a university student who is studying abroad in Japan (This is why he was in the maids outfit when Touma took the picture of him, and in chapter 21, Minjoon said something in korean which resulted in Daiki asking what language he was speaking since Daiki and Minjoon had been only ever communicated in Japanese). Minjoon was speaking in Korean to Daiki while they are doing it and Daiki (in the novel) asked him what he is saying (Since Daiki "only" speaks Japanese, so he doesnt understand minjoon), but Minjoon will lie to him and say it means something else.

[Quote from novel]
[Minjoon opened his mouth, spitting out words in Korean as Daiki’s hand continued to caress his cheek.

“ Ngh... What, are you feeling sorry now? My cheeks aren’t pickled radish. Do you think it’s good to chew like a fucking demon….you asshole . Ugh, please stop already.” (Minjoon says this in korean)

“What are you saying?” (Daikis asked in Japanese)

“Ah…! Haah... I said you’re so sexy.” (Minjoon responded in Japanese)

[Another quote from novel]
[Minjoon began to shout in Korean once again.  

“I’m not the only one who has to learn to be polite, you stupid asshole.” (Minjoon says this in korean)

“Minjoon, what did you just say?” (Daikis asked in Japanese)

“It means I want to be held by you. So please, do it quickly.”(Minjoon responded in Japanese)]

Then one day Daiki will ask Minjoon what he was saying in his sleep. What Minjoon said in his sleep is stuff like "asshole" and "ill kill you Daiki" but in korean. Minjoon notices that Daiki's korean pronunciations are really good and ask if Daiki knows korean. Daikis tells minjoon that he is also part korean.

[Quote from novel]
["My name is Daiki Cho. Is there a japanese man with the family name Cho?”

“You’re korean?”

“To be more exact, I’m a third generation Korean-Japanese.”]

Then Minjoon will be mad at Daiki for a bit because Daiki let Minjoon embarrass himself.

[Quote from novel]
[Minjoon’s shock faded to anger as he remembered all the embarrassing things he had said before. He hated Daiki, who had never uttered a single korean word in all their countless nights together. If he had known before, he never would have used a sentence like ‘I’ll kill you, you asshole’. Moreover, he felt betrayed by Daiki, who had kept silent even after hearing the things he said.]

I just want everyone to remember that Minjoon is Korean, and that the story takes place in Japan where everyone speaks Japanese :) I remember when chapter 21 came out, Minjoon was speaking in Korean to Daiki but the translator didnt put a T/N that said Minjoon was speaking in korean, while the story takes place using Japanese. So this made everyone in the comments confused since many hadnt read the novel, so they had were clueless as to what language was being spoken.
Please read the novel, a fan is translating it from its original language to english for free! The novel will help you understand the story SO MUCH better, and the manga leaves out a lot of important things from the novel that werent added to the manga. Heres the link:
https://archiveofourown.org/works/28126470?view_full_work=true