Title?

I don't think fatales is even a word in French. Fatale means fatal, deadly, or fateful, so there is no plural to those words. Femme of course means woman, femmes means women. Basically the title as they have it translates to "deadlies woman" or "fatals woman." Yeah, it's nonsense, not that they have a lot of Rhodes Scholars tuning in. Good catch.

The correct French is femmes fatales. In French, as in all the Romance languages, a plural noun takes a plural adjective.

Of course, since spoken French drops final consonants, the plural is pronounced exactly the same as the singular. To be clear that you mean more than one woman, you'd use the plural article: Les femmes fatales.