It's 1300. A German-speaking Jewish man is shown a phonically similar sentence rendered in Yiddish with Hebrew characters, and in German with Latin characters. Can we reasonably expect him to comprehend both?
Gotz Aly (Why the Germans, why the Jews) explained that having extremely limited opportunities, European Jews invested heavily in and prioritized education. Whether their own religious education or secular education. As a result, they often had a mastery or at least understanding of multiple languages.
Being persecuted and discriminated against, Jewish people needed the ability to communicate with the Non Jews they were living with. Accordingly, they had to be able to read and talk in the language of their neighbours.