Japanese

I sort of felt that was how Taku expressed his sincerity- Japanese is probably his mother tongue, and maybe him speaking Japanese was him speaking from his heart. Haesoo is clearly so untrusting, and feeling that genuine kindness and comfort from Taku's Japanese was likely why he felt that relief. He may not be able to understand his words, but Taku's demeanor held so much honesty, and that it didn't need any translation.

I had this idea of Haesoo visiting Japan.

Good observation.

Part of his Japanese sentences are praising Haesoo (when they are forthcoming to have sex) but I like 2 final ones the most: It's okay, don't cry and I will hug you. I am still curious why he uses it but no matter what, it's still value to Haesoo.

I have thought about it too. Maybe a short vacation or studying abroad.

Yeah, I always had this thought that later on in the story Haesoo will go to Japan. Maybe going to Japan will be a good thing for Haesoo. He needed it seeing how much he is under pressure in both his personal and professorial life.

I think it's a whole analogy:
Taku speaking a foreign language here is both literal and symbolic. What is the language that Taku is speaking to Haesoo that he never heard from other men before? Kindness/tenderness/respect .... the flashback scenes show Haesoo listening to a language he knew and was used to ( the way other guys and Joowon talked to him and he understood) ... now this guy shows up talking in a language he doesn't know .... The author masterfully puts Taku speaking to Haesoo in foreign language where Japanese represents the other language that Taku speaks to him and it's unknown to him until now.