File descriptions:

images: contains any images required by this language pack
includes: contains language files used by plugins
dialog.css: contains any modifications to the design of dialog windows that might be required to properly display this language
document.css: contains any modifications to the hidden guidelines that might be required to properly display this language
editor.css: contains any modifications to the design of the editor interface that might be required to properly display this language
lang.inc.php: contains core translated language variables and calls to the PHP API

PLEASE NOTE: The $lang variable contained in the translation files is NOT loaded into the global scope. 
The language files are loaded into a class and will not interfere with any $lang variable in your application.