According to her Wikipedia article, she lived between 1478 and 1534 so this is a posthumous portrait.
From Wikimedia. Also posted to the Early 1500s - 1470 to 1550 Album.
This dress has a distinctive retro look to it. The over-dress covers the under-dress or kirtle under a vee neckline in late Middle Ages style.
Keywords: 1537, Cranach - Lucas the Elder, Barbara Jagliellon, Barbara Jagiellonka, Barbara von Polen, księżniczka polska, Herzogin von Sachsen, Jagiellończyk family, Wettin family, German, Steuchlein headdress, neck band, pendant, partlet, square neckline, kirtle, fur-edged over-dress, vee neckline, fur-edged long tight sleeves