Cantonese Chinese: "Wo! Wo!" (Incidentally, the term little children use to describe dogs. Courtesy of melodrame)

Catalan: "bup bup!" (Courtesy of koala)

Czech: "haf haf!" (Courtesy of nYxxie)

Dutch: "waf waf!" or "woef woef." (The 'waf waf' is for smaller dogs that make a somewhat higher pitched bark, 'woef' is for big dogs. Very small dogs do a 'kef kef'. Courtesy of archie2)

English: "woof," "arf," "bow wow", "yip yip / yap yap" for small rat-dogs too.

Estonian: "Auh-auh" (Courtesy of upthorn and "A Guide to Barking Abroad" from his copy of The Best of the Journal of Irreproducable Results. montecarlo says: I'd spell the Estonian entry as "Auhh, auhh", because the "h" is not silent, but pronounced hard, like German "ch" in "Achtung".)

Finnish: "hau hau!" or "vuh vuh!" (Courtesy of Jope and break)

French (from Astérix books. i.e., à la Idéfix - you know him as Dogmatix): "Wouaff! Wouaff!" or "ouah-ouah" (Courtesy of melodrame)

German: "Wau-Wau" (pronounced like "vow") or "Wuff-Wuff". (Courtesy of -brazil- and izubachi.)

Hebrew: The canines of Cana'an supposedly go "hav hav!" , but I can't think of ever hearing a dog emit anything remotely like this. (Courtesy of ariels and undone.)

Japanese: "wan-wan"

Mandarin (Han) Chinese: "wang wang!" (note: the ang is pronounced as an "ah" with an "ng", not like "bang" or "fang". Thanks to blaaf for that one.)

Korean: "mong mong!"(Courtesy of Dystopian Autocrat)

Lebanese: "Haw-haw" (Courtesy of upthorn)

Nigeria - Calabar area: "wai-wai" (Courtesy of upthorn)

Norwegian: "voff voff" (Courtesy of Sverre)

Polish: "how, how" (Courtesy of liontamer)

Portuguese: "au au" (Courtesy of e-anorexia.)

Russian: "gav gav!" (Courtesy of TheLady)

Spanish: "gau gau"(Courtesy of koala)

Spanish (El Salvador): "Guau-Guau" (Courtesy of upthorn")

Spanish (Peru): "gua-gua-gua" (Courtesy of upthorn)

Swedish: "vov vov" (Courtesy of h_e)

Tamil: "Bou-Bou" (rhymes with How How) (Courtesy of rep_movsd)

Thai: "hong-hong" (Courtesy of upthorn)

Ukranian: "Breshe" (Courtesy of upthorn)

Vietnamese: "Gau gau" (Courtesy of upthorn)

Yoruba (Nigeria): "gbogbo" (Courtesy of upthorn)

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.