Is this happening in English or Spanish? I checked the es.po file available at rak's translation server, and it has a correct plurals formula. But if whatever po file you're using doesn't have the right formula to determine plural-ness, this kind of thing happens.

The correct line near the top of the .po file is
"Plural-Forms: nplurals=2; plural=(n != 1);\n"
This is what you should find for most languages.

However, if a translator misconfigured something, or just copied the po header from French, you'd see something like
"Plural-Forms:  nplurals=2; plural=(n > 1);\n"