Berger pointed to mushrooming neighborhoods of Orthodox Jews in Los Angeles, Toronto, and Chicago, with "minivans full of Hasidic kids."

"Among the Hasidim, the growth has been spectacular," Berger noted. "In the 1950s, there were so few of them [in Williamsburg, a neighborhood in New York City's Brooklyn borough] there was only one school and you could fit an entire PTA meeting in a home. That's unthinkable now. There's 300,000 Hasidic Jews in New York alone, and they're growing fast."

The Census points to a curious pattern of aggregation of Yiddish speakers in four metropolitan areas, calling the Yiddish-speaking community "an extreme example of language concentration":

[Seventy-six] percent of all its speakers lived in the New York metro area, with another 6 percent in the Poughkeepsie metro area, 4 percent in the Miami metro area, and 2 percent in the Los Angeles metro area. This means that 88 percent of all Yiddish speakers lived in just one of these four metro areas.

The same report said that many Jewish American Baby Boomers grew up speaking Yiddish at home, but dropped it—along with an Orthodox lifestyle—when they reached adulthood. Berger hypothesized that today's secular Jews just aren't that interested in speaking Yiddish.

"Yiddish will continue to flourish under the Hasidim," he said. "[But] Yiddish as a language of writers will not because the Hasidim don't read those works. They use Yiddish as their lingua franca and to discuss the Torah. They don't read secular works."

The future of Yiddish is a mixed bag, he says: "There will be a substantial population of people speaking it, and they're going to increase. But besides that, not much else [will happen with Yiddish]."

But others in the Yiddish community have a different perspective.

"What we see ... is not nearly as bleak as some people make it out to be," says Jonathan Brent, the executive director of YIVO, which stands for Yidisher Visnshaftlekher Institut, or the Institute for Jewish Research. YIVO hosts a summer Yiddish-immersion program that has seen increasing interest in the language.

In fact, Brent said, the students that come to YIVO to study Yiddish break two stereotypes of students associated with learning the language: They "don't seem terribly religious" (although the organization does not gather data on religiosity), and very few claim Jewish heritage. "Our students come from all over the world: Lithuania, Poland, Ukraine, Germany … they are studying Yiddish because they want contact with this culture that once flourished in their land and are trying to retrieve it."

Yiddish also seems "cool" and involves an element of hipsterism—it's a return to a culture that is at once marginalized but also familiar, says Brent.

"Eastern Europeans and Judaism were marginalized in America by the dominant Christian culture," he said. "But it also entered the mainstream because of media: movies by Groucho Marx, fashion by Levi Strauss … it creates a tension that's exotic but familiar at the same time."