Error selling unique, tradable items

[Rongway]

When selling a unique, but tradable, item (for example, "Plundered Ear Cuffs") to a vendor, the dialogue box warning about selling hard-to-get items says "This item is unique and untradable."
I changed my language to Japanese, and found out it refers to the item as "RARE/EX" instead.

"RARE/EX" is just JP for "unique and untradable". The slash is still an "and" there, and the JP text still wants to mean the same incorrect thing that the EN says, though it seems not to be incorrect compared against the EN text because an English speaker expects a slash to mean "or".

I'm assuming you mean to suggest that the EN text should say "This item is unique/untradable", with the slash to mean "or", and this would make it more accurate in English, yes. It would just have the quirky (but fine for localization) side effect of making it not say what the JP means.