There is a book called “A JuBilee of watching” about Rav Chaim Eliezer Samson, who headed TA in Baltimore. the book mentions that in europe, Yiddish was the language of the balagalos (wagon drivers) but lashon hakodesh was the preferred language of talmidei chachamim. the Chofetz Chaim wrote his scholarly seforim in hebrew, but the ones for the hamon am he wrote in yiddish. My Rosh Hayeshiva ZT”L, who spoke Yiddish natively, once said that people think Yiddish is a lashon hakodesh. It’s not- it’s an “amaleikishe lashon”- ie, a Germanic offshoot. One of the michtavim of Rav Yisroel Salanter ZT”L refers to Yiddish as “jargon”, ie slang. I think the “Loshon hakodesh”ization of Yiddish is part of the beatification of anything to do with Europe. Rav Avigdor Miller mentions ( I believe in “Rejoice O Youth”) that when he first went to the yeshiva in the town of Slabodka in 1932, the nearby city of Kovna had a bus leaving on shabbos every half hour for the mechallelei shabbos to go to work. when he left Slabodka in 1938, a bus left on Shabbos every 5 minutes. This is not to deny that there were plenty of frum yidden in Europe who were niftar al kiddush Hashem, but this is not the picture painted by the current crop of Yeshivish literature about Europe.
There is a gemara where Rabbi Yehuda hanassi (“Rebbe”) asks, why speak Aramaic (the common Jewish language)? either speak Hebrew (the real lashon hakodesh) or Greek (to communicate with goyim. If that’s true for Aramaic, which at least is a Semitic language, kol shekein Yiddish.
There was recently an article in the Jewish Observer by a balabos complaining that people don’t name their kids Chynka, Kreindel, Masha, etc anymore. He asked, “why is your bubbe’s name not good enough for you”? This question can only come from the aforementioned Europe-worship. Kreindel is a German word meaning “crown”. Masha may be a russian form of Miriam, Pesha may come from Penina, Breindel means “brunette”, Goldie means “golden-haired”, Reizel means “rose”, etc. At the time the names were given, the Jews adopted goyishe names or altered jewish names to fit into the local vernacular. Isn’t it better to give a kid an authentically Jewish name like Shlomo than a Germanic corruption like Zalman? Daas chachamim hepech midaas baalabatim- My Rosh Hayeshiva ZT”L said the opposite of this balabos. (I’m a balabos too- no offense meant to other baalei battim). Furthermore, I just saw a study that kids given unpopular names were more likely to commit crimes than kids with normal names. Why subject your kid to ridicule by giving them funny Germanic names? It’s hard enough for kids to stay on the derech today as is.
The one advantage of Yiddish is that it helps keep kids separate from our culture. However, nowadays it’s detrimental to their ability to get a parnassah later in life. There is a story about Reb Yaakov ZT”L (brought down by his son, giving it authenticity) that someone came to him in america, and he didn’t want his son to learn English. So Reb Yaakov ZT”L asked him, how will he function? The balabos answered that Rav chaim Brisker ZT”L didn’t understand Russian. Reb Yaakov ZT”L responded that Reb Chaim had a shamas to read the street signs for him, but the balabos’s son won’t have that. This balabos thought he knew better than one of the gedolim. Sure enough, this sone didn’t stay in learning, and had trouble getting a job because he couldn’t function properly.
As for what is a lashon hakodesh, the Ibn Ezra mentions hebrew, Aramaic, and Arabic and Lshionos hakodesh, all three being Semitic languages (ie related to Hebrew. The Mahara”l says that Aramic is holier than Hebrew. Nowhere does he mention yiddish. Not sure how to shtim the mahara”l with Rebbi Yehuda Nahassi’s statement.