Is this "diet" as in, "diet soda"? Because the picture showed soda but I've never heard of soda just referred to as "diet".
I just posted a similar though when I encountered the translation in the opposite direction, from English to Italian.
I find it entirely plausible that Italian contracts Diet Coke, Diet Pepsi, Diet Soda, etc into a generic "Dieta". However, when translating "It is a diet" from English into Italian, I found myself want to translate the sentence "It is a dietary regimen" instead of "It is a diet beverage".