Mainly, Barty Crouch Jr thought himself to be Voldemort's most faithful Death Eater; it's possible he anointed himself as being worthy of speaking Voldemort's name and as having earned the right to do so.
‘Tell me he told them that I, I alone remained faithful ... prepared to risk everything to deliver to him the one thing he wanted above all ... [Harry Potter].’
Goblet of Fire - page 587 - Bloomsbury - chapter 35, Veritaserum
I personally don't interpret the Polyjuice Potion as being relevant; after saying Voldemort's name in front of Harry, Crouch Jr went on to give a full confession of his plan and actions over the past nine months, and then he attempted to kill Harry, all while still Polyjuiced. I don't think he was concerned about saying Voldemort's name in front of Harry because his intention was to kill Harry; there would be no witness to Crouch Jr saying Voldemort's name if Harry were dead.