In April, the language-learning app Duolingo added its 40th language to its program arsenal: Yiddish. A couple of decades ago, it would have been unthinkable for a mainstream non-Jewish language program to offer an expansive, comprehensive course in Yiddish. But Duolingo’s Yiddish addition only serves to reflect the increased global interest in learning a language that once had as many as 12 million speakers. Ladino, a Romance language of Sephardic Jews still spoken by hundreds of thousands worldwide, has also garnered much interest in recent years. Ladino classes, both online and in-person, are widely available to prospective learners. But while...