Ah you misunderstood you have to set an environment variable, like the PATH environment variable. The name of this variable is LANGUAGE or LANG and its value should be es for Spanish.
I'm not sure why you're seeing a polyglot of UI strings. Try running the libprs500-debug.exe program. That should give you a prompt. At the prompt execute the following code:
Code:
import locale
print locale.getdefaultlocale()