” Moshe Rabbeinu did not speak Yiddish.”
Moshe Rabbeinu didn’t do much of what we do having developed over the eons. One who however diverts from them[ not out of ignorance but out of contempt]
Poretz geder yin’shache’hu nachash
AND
minhag yisroel Kodesh!?
2/Chavtzeles HaSharon (I:OH:10) Loshon HaKodesh is only Kodesh if its used exclusively for holy things. That is once you start using it to speak mundane things, its not holy anymore.
The Rambam writes that a love song in Hebrew is more repulsive to G-d than the same song in Arabic, for instance, because the pollution of the Holy language is an additional crime
3/Though there were Yiddishistin who spoke Yiddish, they took it from us, not vice-versa (as is the case of Modern Hebrew), and as we do not live in a society dominated by Yiddish-speaking frei, there is no benefit of Lo shinu es leshonam by not speaking Yiddish.