One of the best questions that I have seen in a while. This
question can be taken quite literally however one must take into
accounts the different types of duck. For example you have the
mexican duck. Thus ducks speak in different languages.
Quick guide to speaking duck.
Mexican duck -"quack"
translation: eh ese wazzup
French duck -"Queck"
translation: "steeoopid amerwicon"
American duck: "moo"
translation: "quack"