Fiberbabe, i know exactly what you are talking about..

i am terrible with languages other than english and you can keep all your comments about how i mangle english down, please!

but i was in a small chinese take out place, and the father, (chief cook, and bottle washer) barked some orders at his daughter, who was running the register, and tucking napkins and forks into the to go lunch bags.

i looked at the father, and said "She has already put a spoon in the bag, its ok!"-- and he nearly fainted! he said something else, and i replied, "No, i don't speak chinese, i just could understand what you were saying to your daughter.." i don't know how i knew what he had said, but i did get take out all the time, and i guess somehow, my brain had figured that one comment out..

my son does the same thing.. when he went on a work study program in Germany, he "understood" most of the other there, inspite of a very poor command of spanish, and almost know other language..

i used to do it very well with Yidish, (a common language in my childhood nieghborhood) but i do it less and less well with age.