If you need to add new language to the application, you will need to create your own language files.
Please create a copy of the English (en) language files:
Then, rename them accordingly to your new language. If you review the file \library\Zend\Locale\Data\Translation.php and search your new language on the array $languageTranslation you will find a code for the language, for example: 'Serbian' => 'sr' so you need to rename the new files using the code of the new language, for example:
Now, it is time to start the translations. You will need to open the file \language\core-sr.po and add your translations there. You can use a free editor like Poedit.
Any time you change the file \language\core-sr.po, you need to convert it to \language\core-sr.mo, which is the binary file that Clientexec uses to load translations. If you are using Poedit you can easily generate the .mo file from there. If you are not using Poedit then we have made a tutorial video on how to generate a .mo file: http://www.screencast.com/t/h2zAKFcKi
The video just shows the following: