In Weimar, Germany, where the streets once echoed with the notes and ideas of Bach, Liszt and Goethe, another European musical tradition has been reverberating for more than a quarter-century: Yiddish.The annual Yiddish Summer Weimar festival brings hundreds of musicians, students and scholars to the city — most famous for the ill-fated republic that preceded Nazi Germany — for workshops, concerts and performances celebrating the language, music and culture of Ashkenazi Jews — a culture the Nazis sought to destroy but that has continued to reinvent itself. Since 1998, the program has both grown alongside and influenced the growth of...