I would say that the fact that the boocklet doesn't say anything about it doesn't automatically mean that it is allowed to speak. I take it to mean that the players should agree before each game whether they will be allowed speak or not. It is good to alternate both ways and have different experiences of Mascarade.
We houserule a strict no interference rule when it comes to other players' actions. It was after one player said that they'd only challenge another player's claim if no other player was planning to challenge, which then led to some players discussing which of them would be the sole one to challenge and collectively deciding who had the best shot of having the required card. It all felt a little bit like it was taking away from the game's design.