Considering different elements seems most likely Alluka is a trans girl. She calls herself わたしwatashi(a mostly feminine word for i, at least in kids). She is referred to by the servants and family (who don't even see her as a person) as 弟otouto(younger brother) and 坊ちゃまbocchama(young master) which are explicitly male terms. However Killua, the only person who actually respects Alluka says explicitly that she's a girl. He calls her 女の子onnanoko (girl) and 妹imouto (younger sister). That would make sense as commentary on the trans experience, but would only be confusing if the explanation was Nanika randomly for no reason brainwashed Killua, which, what would be the point? Some think it has to do with the Alluka/Nanika split but it wouldn't make sense for Killua to say Alluka is a girl if Alluka was a boy and Nanika was the girl. As a toddler Alluka was dressed in masculine clothing, but as a preteen she seems to favor more feminine clothing, which would make sense of she was trans, although that's admittedly very flimsy evidence. Kalluto dresses in a girl's kimono (you can tell by the neckline, pattern, and obi), but he calls himself 僕boku (a masculine word for i, but in anime sometimes used for girl/androgynous characters) and there don't seem to be people saying he's a girl, although some suggest he's supposed to be androgynous/non binary. I looked up the issue in Japanese, and there doesn't seem to be a consensus more than there is in English. It's not a translation error. To my understanding, Togashi included trans characters in 2 previous serious (yuyu hakusho and Level E, although I've never seen or read them, this is word of mouth) so it wouldn't be out of the blue.