Sorry, but Tartanwizard is right about Israelis speaking modern Hebrew, but wrong about modern European and American Jews speaking Yiddish. Only a very tiny minority of Jews now understand that language - and then only as a secondary language (similar to lowland Scots who know Gaelic) - though it was certainly widely spoken by Jews a century ago. Outside Israel, most Jews speak the language of their own country, just the same as Protestants, Catholics etc do.