Chuchu looked at Ace with confusion and fear before she looked at Cyrus, who was sitting on Ace's shoulder with a wronged look, and uttered, "Chu.. chu…(Boss, what is your brother saying? He's very scary!)"
Ace suddenly realized that Chuchu was speaking in the beast language, not the common tongue, which meant she didn't understand what he just said. Since she might've never encountered humans, she didn't know their language.
Understanding his mistake, he repeats what he said in beast language for Chuchu without giving Cyrus any chance to speak first. After all, if he wanted to appear as Cyrus' big brother, he needed to make sure Chuchu understood he could communicate with her, or the intent behind his words would have no meaning if Cyrus repeated them.