sammygol, what does that position have to do with this? I hope you aren’t trying to discredit the Lev HaIvri based on something else he was opposed on. Reb Moshe to disagreed with many on Shabbos Clocks. So what?
The Kuzari writes that Avrohom Avinu, therefore, spoke 2 different languages. One for holy speech – that was Loshon HaKodesh, and the other for mundane speech – that, the Kuzari says was some non-Jewish language that Avrohom Avinu took and changed around a little on his own. And thats the idea behind Yiddish. It is a non-Jewish language that we took and twisted a bit in order to make it exclusive among us.
The Chasam Sofer EH 2:11 says that in ancient times Jews used to use a modified version of the non-Jewish languages for everyday (divrei chol) talk, similar to what Yiddish is.