Avi K – I have never asked them; and it is almost irrelevant to this discussion. If you are discussing the advantages of knowing or understanding Yiddish, the fact that you need it to communicate in some of the worlds best yeshivas is good enough for a ben Torah.
Why they don’t speak English? Most of them don’t understand it at all. If you are brought up in a totally isolated community in Yerushalayim by parents who come from Poland, the likelihood is that you don’t speak English. Some of the hierarchy have picked up a bit of English (one of them married a London girl), but the way they learn is in Yiddish. If you want to ask a question in Yiddish, you can. Nobody will complain or say a word. But many feel uncomfortable doing so. You can speak to them in Ivrit, surprisingly.