If Yiddish is holy because Jews used it in Europe, then outdoor outhouses are holy as well, along with spittoons. Both of these were prominent features of life in Eastern Europe.
Plenty of people argue on the Mishna Brura if they have another big source backing them up. The problem is, the Aruch Hashulchan doesn’t argue. Neither does the Shulchan Aruch HaRav. And Reb Moshe emphatically agrees to the mishna brura, saying there’s no difference between Yiddish names and English names in halacha when it comes to gittin. The Rema paskens it’s ok to read secular books in Lashon haKodesh. if Yiddish was holy then there would be a similar psak for secular books in Yiddish on Shabbos. Nisht da kein psak. I know the Divrei Yoel said otherwise- he said Yiddish is heilig vayl durch yiddish mir zennen farshidit fun di goyim. But the stimas haposkim overwhelmingly says otherwise.
Rav YB Soloveitchik switched from Yiddish to English when the bochurim no longer understood Yiddish. I was told by someone who listened to his shiurim that they got noticeably watered down once Rav YB Soloveitchik switched to English, as it wasn’t his natural language, and he had to focus on saying his Torah in English rather than the Torah itself.