“Butter wouldn't melt in someone's mouth”in other languages

Same idea, different pictures

English pictures a person so cool and prim that even butter wouldn't melt in their mouth. Japanese wears a cat on its head, neko wo kaburu, hiding its true nature under something meek.

French has two. Sainte Nitouche is not a real saint: the name is n'y touche, “doesn't touch it”, said of a woman who acts as if she'd never touch anything. On lui donnerait le Bon Dieu sans confession means he looks so innocent you'd give him communion without hearing his confession first.