The meforshim say Hebrew is a lashon hakodesh. The Maharal (who lived at the time of “lashon ashkenaz”, a protoyiddish, added Aramic. He does NOT say Yiddish. The Kav Hayashar transltated his mussar sefer into “Lashon Ashkenaz” so the women could read it, but never said it was holy. In fatc, he says reading Lashon Hakodesh is good for ruchnius, but doesn’t say reading “lashon ashkenaz” is.
The Igros Moshe says the only difference between “Ettel” and “Ethel” is that it’s too late to protest “Ettel”.
Yiddish is a form of German. Lashon Hakodesh is holy. The Chazon Ish gave his haskama to teaching in Ivrit. The bundists insisted on only speaking Yiddish. I’d rather be like the Chazon Ish than a Bundist. Adding a mitzvah of speaking Yiddish is Bal Tosif. This was the one mitzvah of the bundists, but it’s not in my Torah.