i think germans can understand yiddish if they try a bit. a bunch of new words, some of the grammar's different. basically yet another german dialect. you'd have to learn the hebrew letters to read it though.

like, two people speaking fluent yiddish next to you might stump you if you've never heard it before, but if you meet a yiddish speaker and you both want to communicate, it's bound to work (and get progressively better very fast). italians and spanish speakers can do the same, more or less.