I’m chiming in here in the middle of a long running discussion, so I don’t know if this has already been mentioned. I heard in a va’ad given by the late Lakewood mashgiach Reb Nosson Wachtfogel zt”l (this is going back about 36 years or so) that the Rishonim sanctified (“mekadesh”) Yiddish in a manner similar to what the Chachmei HaTorah did with Aramaic at the beginning of the Bayis Sheni era. The va’ad was of course given in Yiddish. The mashgiach zt”l’s attitude toward English was well known. Although he spoke it fluently, he used to refer to it as “Goyish.” As well it really is.
Rav Avigdor Miller zt”l’s attitude toward Yiddish is likewise well known, and I think this has already been mentioned by other posters. I did hear him (on a taped shiur) say that Yiddish is surely worth preserving. But only if it makes sense in the context of your particular family. If the parents don’t speak it, but are adamant that their children will, and that they should learn in Yiddish speaking schools, well, they could be asking for unnecessary problems. I think Rabbi Miller gave an example when beginning to teach an English speaking child Chumash, and the child encounters the first passuk in the Torah: “Bereishis bara Elokim es ha’shomayim ve’es ha’aretz.” And the rebbe “teiches” it: “In onheib hot de Eibishter ba’shaffen die himmel un die erd.” So the child doesn’t understand the original or the translation! So what have we achieved. This is often a recipe for future educational disasters, r”l.
The matter of Ivrit is a bit complicated. While it’s true that it’s largely based upon Lashon Hakodesh, it most definitely is NOT. You can refer to the machlokes between the Rambam and the Ramban (mentioned in the Ramban’s commentary to Parashas Ki Sisa) as to why Lashon Hakodesh indeed has that appellation. It’s pretty clear that according to both opinions Ivrit doesn’t qualify.
A hundred years ago, the matter of Ivrit vs. Yiddish was a major source of contention, primarily in Eretz Yisrael. Since at that time, Ivrit was being used as a means of tearing Jews away from Yiddishkeit, it’s undsrstandable why the Gedolei Yerushalayim fought tooth and nail against Ivrit.
Having said that, it’s worth realizing that except for some Chassidic groups and some Yerushalmi kellilos, the vast majority of chareidim in Eretz Yisrael today speak Ivrit at home, in yeshiva/kollel/BY, etc. This seems to be primarily the outcome of the Chazon Ish’s decision to go with Ivrit in the Yishuv HaChadash communities (i.e., everyone except the Yerushalmi “Yishuv HaYashan” who still hold strongly to Yiddish).
I suppose we could sum it up by saying that if you’re Ashkenazi and Yiddish is possible and practical, it’s worth making the effort to preserve it. It is a barrier against the deluge of secular “culture” that now inundates us all more than ever before. If it will lead to obsssivity, neurotic children in school, difficulties in shalom bayis, or who knows what else, it’s not an issue. English and/or Ivrit are acceptable without any guilt trips or feelings of inferiority. Serving Hashem is what counts. There’s not the slightest problem if you are more comfortable doing it in a language other than Yiddish.