Unzip to the [WizTree]\locale folder (normally C:\Program Files\WizTree)

The folder structure for translation files is
[WizTree]\locale\XX\LC_MESSAGES\default.mo
[WizTree]\locale\XX\LC_MESSAGES\default.po

where XX is the language code

Restart WizTree, then select Options->Language and select the language. 
If WizTree does not detect the new language check that you're using the correct folder structure.