According to the rabbis, the whole of the Hebrew slave people were careful to maintain their own language. The sons of Jacob brought Hebrew to Egypt with them, and the tribes departed after so many generations still able to speak Hebrew.
Centuries later, Jews from Eastern Europe brought Yiddish with them to their new abodes in the United States, Australia and elsewhere, and Jewish meetings abounded with passionate speeches in Yiddish even when the younger generations hardly if at all understood the language.
But the use of Yiddish was maintained, as its proponents used to say, “Because this is the Jewish language, and we are Jews”.
Not that this means that every Eastern European Jew was comfortable with Yiddish.
The story is told of Rabbi Shmuel Mohilever who convened a conference of Russian Jews in 1897 to draw up practical proposals for the creation of a Jewish homeland.
Rabbi Mohilever insisted that all the speeches be in Yiddish, even though some of the delegates were much happier with Russian.
To their protests the rabbi replied, “If you speak in Russian you will wax eloquent and we will all enjoy it, but there will be little practical effect. On the other hand, if you have to work hard to give a speech in Yiddish, you will make sure your points are clear and terse, and we will get some results”.
Moses of course did not know any Yiddish, and it may be that his Hebrew had become rusty during his years in the Egyptian court, but when he did give a Hebrew speech every word was weighed, every word was a jewel, and he was able to achieve what better Hebrew speakers might have been unable to put clearly and succinctly.