Jothar:
Chasam Sofer EH 2:11
Kuzari
Maharil Diskin: “For generations we are accostomed to speaking Yiddish, not Loshon HaKodesh.”
Sefer Lev HaIvri (Hagaon Rav Akiva Yosef Shlezinger) says we should not change our spoken tongue from Yiddish, and he draws parallels with our usage of Yiddish in modern times, to our usage of Aramaic in ancient times.