That’s like saying “frankfurter” isn’t an English word because it came into the language from German. “Tamal” today is certainly a Spanish word, even if it originated in Nathuatl (tamalli); and “tamale” is an English word, even if it originated in Spanish.
Wiktionary says that tamale exists in Spanish dialect, and there is some indication that it exists in Tex-Mex. In English it would appear to be a back formation from the Spanish plural tamales. In Spanish dialect, especially ones used in the US like Tex-Mex, it could have been adopted from English; or perhaps less likely is a survival of the Nahuatl tamalli.
I’m in southern New Mexico. Around here, “tamales” is plural, “tamal” is singular, and “tamale” is the gringo word for “tamal”.
We do the same thing with “malpais”. It’s actually the singular form (the plural is “malpaices”), but we see the “s” at the end and assume it must be plural. So you will hear people talking about “a malpai”.
I knew about “tamale” but hadn’t heard that about malpais (there aren’t any malpaises here on the east coast). The same thing has happened with Greek: “a kudo.” So many Greek & Latin words end in -s, it’s a puzzle this hasn’t happened to more of them.
*“Look at the big tent—must be a circu in town.”
*“Can you give me a statu on that project?”