PoEdit let you easily generate a ".mo" file from a ".po" file which you can get from the official concrete5 mygengo language file archive. The ".mo" file is needed so that you can use the language with concrete5 if there is no special customized version of concrete5 in your desired language or if it is outdated.

Download Poedit here for Windows, OSX and Linux: http://www.poedit.net/download.php

1. Download the desired language zip file from http://mygengo.com/string/p/concrete5-1/
2. Extract the files
3. Rename the desired version file (e.g. "51.po") to "messages.po"
4. Launch PoEdit
5. Open the "messages.po" file with PoEdit
6. Save the file in PoEdit (PoEdit now compiles the .mo file)

There will be a new file called "messages.mo" in the same directory where your "messages.po" is located.

You have to create two directories now in your concrete5 installation:

PATH_TO_YOUR_C5/languages/LANGUAGE_CODE/

e.g. german: PATH_TO_YOUR_C5/languages/de_DE/

In this new folder you create a folder called "LC_MESSAGES":

PATH_TO_YOUR_C5/languages/LANGUAGE_CODE/LC_MESSAGES/

e.g. german: PATH_TO_YOUR_C5/LANGUAGE_CODE/de_DE/LC_MESSAGES/

Now copy the generated .mo file in the new directory:

PATH_TO_YOUR_C5/languages/LANGUAGE_CODE/LC_MESSAGES/messages.mo

If your website is running a version of concrete5 prior to 5.4.2, you'll need to add a reference to this language within your config/site.php file. Add this PHP within the file:

define('LOCALE', 'de_DE');

If you're running concrete5 5.4.2 or greater, you can make this change through the dashboard. Login to your concrete5 installation as an administrator.

Visit http://yoursite.com/index.php/dashboard/system/basics/multilingual/ The language you downloaded should appear as available in the default language dropdown. Select it and save your selection.

Some Contents copied from official download page: http://www.concrete5.org/developers/translate/