It looks like the Greek version is the point where 'almah' got translated as 'parthenos', a word that more specifically means virgin.

It's definitely become a key part of the tradition. I'm now agnostic, but went to a Catholic school growing up, so I know well how much importance is placed on it. I've just always found it interesting how such a core belief of the most popular religion in history might (or might not) have been based on a simple mistranslation.