Dutch:
"Tjilp-Tjilp" (Courtesy of SharQ.)
English: "tweet," "chirp-chirp."
Finnish: "Tsirp tsirp," "tiu tiu" (and several more, depending on the species. Courtesy of Jope.)
French: "cui-cui"
German: "piep", "tschilp", "zwitscher" and similar cutesy things, depending on species (cuckoos, for example, say "Kuckuck"...)" (Courtesy of gwenllian.)
Greek: "tsiou" (Courtesy of donnaidh_sidhe.)
Hebrew: tsif-tsif (Courtesy of undone.)
Japanese: "pi-pi"
Norwegian: "pip pip", "kvitre" (Courtesy of Sverre)
Portuguese: "piu-piu" (Courtesy of e-anorexia).)
Spanish: "pio pio" (Courtesy of koala)
Swedish: "pip pip" (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.