Dutch:
"Boe" (Courtesy of skow)
English: "moo"
Estonian: "mop"
Finnish: "muu," "ammuu" (Courtesy of Jope)
French: "meug, meugle"
German: "Muh" (Courtesy of ba)
Greek: "mu" (Just kidding)
Hebrew: moo (Courtesy of undone.)
Hungarian: "mú" (Courtesy of ba)
Japanese: "moh"
Norwegian: "Mø" (Courtesy of Sverre)
Polish: "Yazooo"
Portuguese: "múúú" (Courtesy of e-anorexia)
Spanish: "muu"
Swedish: "muuu" (Courtesy of rollo)
please /msg p_i with further contributions to this (or other aspects to noises animals make in different languages), assuredly the best use yet found for the global forum that is the Internet.