“Mir kumen on, mir kumen on! Un fest un zikher undzer trot!” Late on a recent Friday night, dozens of voices joined in this Yiddish anthem — “We are coming, we are coming! And our step is firm and true!” — and soared from a conference center among gum trees and kookaburras outside Melbourne, Australia. Today, Yiddish is most commonly used in ultra-Orthodox communities in places like Brooklyn or Jerusalem. But in Melbourne, snatches of it can be heard on certain streets, around multigenerational dinner tables, on stages and in classrooms. And one weekend a year, Australian speakers of Yiddish...