Okay, I just have to get this off my chest:
Jewish: Of or relating to the Jews or their culture or religion.
Yiddish: The language historically of Ashkenazic Jews of Central and Eastern Europe, resulting from a fusion of elements derived principally from medieval German dialects and secondarily from Hebrew and Aramaic, various Slavic languages, and Old French and Old Italian.
You see, there's a difference.
So, when someone asks me to "say something in Jewish," I always answer, "bring a little sweater."