As pointed out , no halacha sefer gives Yiddish a status of a holy tongue lehalacha. it is a lashon chol like English. The only question is if it’s holier because it has lashon kodesh and phrases that hint at Torah.
The question is, what’s the practical difference, even for those who claim it’s holy? Let’s go through each possibility.
1. It takes between 500 and 1,000 hours to learn a language. If a man asks a posek if he had choice between learning Torah and a language which one should he do, even those who claim Yiddish is holy would admit you should learn Torah instead. Actual Torah >Yiddish, even if you say it’s holy.
2. If a woman asks , she will be told to learn lashn koidesh instead since it will help out her davening. Yiddish is nice, but a practical language is better. If she already knows Lashon kodesh, she would be better off learning Spanish or something practical. Or she should laern Tanach.
3. What about teaching Chumash in Yiddish to kids who don’t speak a word of Yiddish, and neither do their parents or Rabbeim? This is obviously ridiculous. There is a chiyuv to teach kids torah. teaching them in Yiddish ensures they don’t understand and is Bittul Torah, forgetting about the terrible chinuch of making kids unhappy in school over a narishkeit. Whatever mitzvah kklusha there is in Yiddish becomes a mitzvah habaah beaveira over the bittul Torah caused by teaching kids in a language they don’t understand. If we’re not mevateil tinokos shel beis rabban over Binyan beis hamikdash, surely we’re not mevateil them over Yiddish. There are those who insist that “the kids pick it up” but practical experience shows it’s not true except by a few iluyim. And even if they pick it up, how much bittul Torah happened before they picked it up? And if you insist on teaching kids in another language, you are better doing Hebrew ivris like the beis yaakovs do, as the ivris will help the kids understand Tefilla and Torah.
I am aware of a few schools that do teach American English-speaking kids Chumash in Yiddish, but I can’t imagine any daas Torah advised this stupidity. It’s pashut bittul Torah.
4. The only possible nafka mina to this whole debate is with kids who come from a a bilingual home and are fluent in both, though the kids get easily confused as to which language is which. I still remember hearing a kid ask “How do you say ‘apple’ in English?” While this is relevant in Chassidish circles, it’s absolutely pointless in litvish circles, where Yiddish is just an academic “shtultz shprach” but nobody practically speaks it anymore. Many shiurim in Eretz Yisroel have already switched to ivrit since giving them in Yiddish is a waste of time. Expect that trend to continue.
I also admit that if you ask a poseik should you spend 500-1000 hours on Facebook and other social media sites like ywn cr or learn Yiddish, he would tell you to learn Yiddish if you’re not going to learn, if only to be productive.