Nellie and Percival's son is named Benjamin after the grandfather, although the Cohens are Europeon Jews, who normally name after the dead and find it extremely unlucky to name after the living. Jews in the later half of the 1800s were of German origin and did not speak Yiddish, although the Cohens do. The babies were not Jewish either way; the Jewish religion goes by the mother. Considering the Cohens took the intermarriage so well, it is strange they expect the babies to be Jewish.
German Jews were more modern and free-thinking than the Cohens were portrayed.