The language directory (found in your MPN root) contains the file(s) that store all the translated phrases. The default translation file used by MPN is the "template.php". This is the file you should use as a base reference when doing your translations.
Now your ready to do some translating, so create a copy of the template.php and name it according to the language you are using. The naming convention for language files is: lang-xxxxxxxxx.php (where xxxxxxxxxx is substituted by the name of your language). Use a text editor to edit this file.