Open your browser and navigate to http://mygengo.com/string/p/concrete5-1.

Look for the language you want to download and get the file and extract it into a local directory.

Rename the file matching your concrete5 version to messages.po.

Go to http://www.poedit.net/, download Poedit, and install it.

Open the messages.po file you've downloaded using Poedit.

Save the file in Poedit; this will generate a file named messages.mo in the same directory as messages.po. You should have a messages.mo file now. If you've downloaded de_DE you have to put messages.po in the directory languages\de_DE\LC_MESSAGES. You have to create the subdirectories manually.

Once the files are where they belong, open config/site.php in the text editor of your choice.

Insert the line define('LOCALE', 'de_DE');. It should look like the following:

<?php 
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'concrete5');
define('DB_PASSWORD', ...