Ames, I think you hit the nail on the head.
There are 613 mitzvos in the Torah, many more derabbanans, and none of them are “speak Yiddish” (at least not in my shulchan aruch- if someone here has a mekor for this alleged mitzvah, I’m still waiting to see it cited).
Hashem will judge me in shimayim based on my mitzvos and aveiros, both bein adam lamakom and bein adam lachaveiro. I will not be asked if I spoke an offshoot of High German when I learned Torah. In fact, the ikkar (main part of the mitzvah) of Torah Shebaal peh is havana- understanding. According to many poskim, if one learns gemara but doesn’t understand it then one is not Yotzei the mitzvah (one has not fulfilled his obligation). If a speaker speaks in Yiddish, preventing me from understanding his Divrei Torah, then he just wasted my time. If your words are important enough to be shared in public, then aren’t they important enough to be shared to everyone? Chazal used to have a meturgeman to repeat and translate, to make sure everyone understood. They didn’t say “If he doesn’t understand that’s his problem”.