A word that Spanish speakers use for cereal — and it is very widespread — is "cornfleik." The word "cereal" has multiple senses, but for the one that refers to breakfast cereal. The one that just means grain should be "cereal" in Spanish, I suppose. But yes, regarding "cornfleik," I'm pretty sure that I've seen it on supermarket aisle signs in Spanish-speaking places. To denote "cereal." Once, my Nicaraguan host dad, speaking on the phone as I ate a bowl of cereal (a mix of Cinnamon Toast Crunch and Cheerios), said replying to my host mom's question about what I was doing, "No, nada. Tá comiendo cornfleik."