The closest way to write the French word “mien” phonetically in Japanese would most likely be as “mia”; “mien” in Japanese phonetics would be the equivalent of the French “mienne” – a misgendering of the word. (There really is no equivalent of the nasal sound on the end of “mien” in Japanese.) So I’m guessing it was originally written phonetically in Japanese, and the translators then just transcribed it as written.
Hum, I don't agree with you tho, they literally can say and write "mien" katakana (ミイン). They have the syllabical and non syllabical "miミ"/"eイ"/"nン", they have a syllabical language, if you cut the the word then it's "mi-e-n" (ミ-イ-ン) and if they say it literally then they say it exactly like a french. "Mienne" on the other hand would be more difficult yeah, they'll probably say something like "mi-e-ne" wich could be correct but they would put a phonetic accent on the "ne" at the end which would be a mistake but with an effort they could totally say it. Japanese can speak french but their difficulty would lie on our prononciation of the "r" and in general with one consonant directly after another like "FRance" they would automatically without any effort say the well known "furansu" but with a little effort (like everybody else to talk another language) they can do it.
But, maybe you're right and it were originally writen like this, but given how japanase are a little too fan of french, I find this mistake weird, or maybe the artist/scenarist made a mistake (and her/his editor didn't see it either...)
But “ミイン” is actually the phonetic equivalent of “mienne”; the “n” is silent in “mien.” I think it’s a received auditory phonetic spelling that leads to the confusion. After all, if his name were to be Italian, it should be “Mio,” not “Mia.”
This manga describes the Venus de Milo as “unfinished” as opposed to what it clearly is – damaged – so it’s pretty obvious the mangaka didn’t do a lot of in-depth cultural research.
Not my cup of tea tho but whatever. BUT ! "Mia is the frend word for "mine"" as a french I was like "bwahaha no, wth ? XD that's Italian but ok." I laughed really hard, I have no idea if it's the author's mistake or the traduction team.