Languages for WordPress: Where are the .po files?

I’m not sure whether this is a plugin question, since the language pack is not really a plugin, but anyway.

In the description, it is said that the package includes the .po files for easy further translation:

Of course, we’re not that multilingual though, so we’ve included both .po and .mo files so you can edit and improve them to your hearts content.

I’ve just downloaded the package, however, and there are only .mo files in there.