I'm bilingual, but it depends on who. If it's a native person giving me the text, I wouldn't suspect anything, but if a Japanese person gives me a whole bunch of text and it simply reads too well, then I would assume it was written by ChatGPT or translated by DeepL.

I don’t use ChatGPT very much, but I wouldn’t be surprised if it’s a lot better at generating natural English than Japanese.

There’s simply more data that chatGPT has learned from for it to be able to generate passable English.

I think there is also an element of perfectionism in that everyone who speaks English is use to bad English, but there aren’t as many bad Japanese speaker and there is a lower tolerance for slightly off Japanese.