"Nine" joke

I was meeting a friend in a bar, and as I went in, I noticed two pretty girls looking at me.
“Nine, ” I heard one whisper as I passed. Feeling pleased with myself, I swaggered over to my buddy and told him a girl had just rated me a nine out of ten.
“I don’t want to ruin it for you, ” he said, “but when I walked in, they were speaking German. ”