Derek Abbott's Animal Noise Page tells you what the standard onomatopoeia is for animals in different languages. Interesting how a cuckoo basically says "cuckoo" in all languages shown! Snakes also pretty much all say "ssss."

There's also a fascinating section on animal commands, so you can find out how to say "giddyap," "whoa," "here kitty kitty" and "scat" in different languages.

EDIT: For more fun and to hear people actually saying the onomatapoeia [not just for animals], go to bzzzpeek. It's fascinating!