Ashkenazi religious life ran on a division of labour between languages, and Yiddish had a defined half of it. The fixed liturgy stayed in Hebrew and Aramaic. So did the legal literature, the responsa and the commentaries. Yiddish did everything else: it explained those texts, taught them to children, preached from them, and told stories about the people who had lived by them.
The verb taytshn means to translate or to gloss, and it names the classroom method by which a boy learned scripture, one Hebrew word at a time answered by its Yiddish equivalent in a fixed chant. The habit produced a whole translation register, wooden and word-for-word, which readers recognised as a language of study rather than of speech.
Out of that register came the best selling Jewish book of Ashkenaz. The Tsene-rene retells the weekly Torah portion in Yiddish with midrash and moral commentary woven in, and it was printed and reprinted from the turn of the seventeenth century onward. It was read at home, especially by women, who were not taught the Hebrew curriculum, and it shaped what most Ashkenazi Jews actually knew of the biblical narrative. Beside it stood the tkhines, supplicatory prayers composed in Yiddish for occasions the fixed liturgy did not cover, many of them written for and some by women.
Hasidism, from the eighteenth century, prayed in Hebrew and preached in Yiddish, and made the Yiddish tale a religious form in its own right. That arrangement has not lapsed. In Hasidic communities today Yiddish is still the language of instruction, of the sermon and of ordinary life, while the prayers are said in the older tongues.