I couldn't actually find any English-Yiddish online dictionary, any more than weepiper could evidently. Just sites with jokes (good jokes), and glossaries of yiddish phrases and so forth.

Threre's a story, probably in one of Leo Rosten's books, about a grandmother on a bus in Tel Aviv with her little grandson. He keeps on pointing out things excitedly, and chattering away in Hebrew - and the granny keeps on saying "Speak Yiddish". A passenger asks her why she keeps doing that and she says "I don't want him growing up forgetting he's a Jew."