I'm French and reading your author note made me react : Décolletage, that just doesn't exists !

So I just check one of the biggest French dictionary. "Décolletage" does exist but has nothing to do with female anatomy or clothing. It is a technical term used in agriculture or in nut-and-bolt works with tow different meanings. Nothing to do with your story.

However, "décolleté" is the correct term you need. It can be either a name or and adjective. As a name it denotes the female anatomy part as you explained in your note. As an adjective, it denotes the part of clothing which lets you see the corresponding female anatomy part.