If you want to help us translating Concrete CMS, you can ask to join a translation group.
First of all, you need to Sign In, then simply go to the Translation teams page, choose your preferred language and hit the Join this team button. You'll receive an email when your join request will be accepted.
Once you'll be part of a translation team, go to the Translate page, and click the Translate button.
If you want to download the translations files, go to the Translate page.
You can choose a specific Concrete CMS version in the dropdown menu.
Locate the language that you need, and click the mo button to download the related language file.
Then you can save the downloaded .mo file to the appropriate location (for instance:
application/languages/it_IT/LC_MESSAGES/messages.mo for Italian).