Where is the right .mo file inside the language repository?

To translate you need few things first in place.

1. Create a directory called languages inside wp-content .
2. Now define your country and locale in wp-config.php file. For example as you wish to translate into italian. You should add these lines.
   define (‘WPLANG’, ‘it_IT’);
3. Now as you have read on forums you need to put the .po file in languages directory
   for example This file.

This should solve your problem.