Not at all more my English ignorance than yours! Is "Decal" An American saying?
It's in common use in German modelmaking parlance, having been popularized by Revell some decades ago. AFAIK, the term was transferred from English which in turn got it from French "décalcomanie" which is a noun for the "process of transferring sth.". I'd write it off as an example of "noble vs. peasant speak" (e.g. beef from cow), if transfer wasn't a latin import itself.

I also used "decals" exclusively in English (in German, there are two equivalents, "Nassschiebebild" for wet transfers/decals, and "Trockenabziehbild" for dry, "rub-on" transfers), but pretty much no one uses those except when a technical description is called for.