I believe it was here:
http://www.oddworld.com/universe/wil...1/ow_cg01.html
And yes, the singular of 'ratz' is 'rat'.
The creatures of Oddworld do not speak English. They speak whichever language the people speak in the country that they're sold. In Spain they speak Spanish, in France they speak French, et cetera.
Another reason is that it was designed by Americans (who, as I'm sure you knew, speak English).