It doesn't make sense to people who do not know what the words mean so it lessens the accessibility of their songs
Ponzoña, Que sera sera, Hwaa - I didn't know what any of these meant at first, I know now that I looked them up, but it's not a good first impression when you just don't know what the title means
For people who don't read Korean: "쩔어" makes no impression
For people who don't read English: "Dope" makes no impression
I think since you can generally only title a song in one language (and alienate everyone who doesn't speak that language unless the phrase is well known), the best case scenario is to only title songs in Korean, so that people who live in South Korea would get it
People have made it normal to have pure English titles, but I think it's logically more correct to only have Korean titles
Edit I forgot a lot of Kpop songs have a Korean title and an English title, but let's say if they only wanted to have 1 title, they should make it a Korean name
Edit again : but what they should not do is have a title that's only in English or only in Spanish and it's not a common phrase that everyone knows, so not even people who live in South Korea can understand it