Well, the question is “Which Indo-European language, spoken by many Jews mostly around the Mediterranean, is derived from Old Spanish but is not Castilian nor Catalan nor any other language that most Americans have heard of?”
The answer, as you have seen in the title to this post, is Ladino, also known as Judaeo-Spanish or גﬞודﬞיאו־איספאנייול. As it has about 60,000 native speakers and over 100,000 non-native speakers, one wonders why so few Americans have even heard of it. Most Americans, after all, have heard of Yiddish; and Ladino plays a role for Sephardic Jews that is more-or-less parallel to the one that Yiddish plays for Ashkenazi Jews. And therein lies the two-fold answer. First, there are still over a million native speakers of Yiddish worldwide; second, there are far more Ashkenazi Jews in the U.S. than there are Sephardic Jews.
Just as Yiddish is pretty much a dialect of medieval German with an admixture of Hebrew, English, Polish, and Russian, so is Ladino pretty much a dialect of Renaissance Spanish with an admixture of Hebrew, Portuguese, Persian, and Turkish. I don’t claim to have more than a teaspoonful of knowledge of Ladino; I was inspired by listening to a segment on the podcast Unorthodox that included some beautiful Ladino singing by Sarah Oeste, and then I did a little bit of linguistic research to follow up.
Here is a brief excerpt from a Ladino song, so you can at least see a bit of what the language looks like in the Roman alphabet:
Quando el Rey Nimrod al campo salía
mirava en el cielo y en la estrellería
vido una luz santa en la djudería
que havía de nascer Avraham Avinu.
As you can tell, the connection with Spanish is immediately obvious.
Unfortunately, there is a lot of evidence that Ladino may be a dying language. But many people are working hard to prevent that from happening