Another fix for nsis-msgfmt mapping

* po/nsis-msgfmt.c: Map Chinese simplified and Traditional differently.

Still not sure what I am doing here but wihtout this I get warnings
that zh_CN and zh_TW is not used and the strings are mapped to
language 0. With this mapping it appears to work. I've taken
nsis.sourceforge.net/Examples/languages.nsi as an example.