Every Sunday morning, a group of families in Stockholm, Sweden, meets in a local school to create handicrafts, do gymnastics — and learn Yiddish. Katka Mazurczak, the instructor of this grassroots group called The Yiddish Club, told me that the families seem to really enjoy the weekly Yiddish immersion. One of the resources she uses is YiddishPOP, a free online animated and game-based learning tool that features short episodes about a young teen…