The language files are well commented and easy to customize the text displayed for any of the core glFusion screens. Plugins are also expected to use a language file for their user screens. A language subdirectory with the distributed plugin is where the different language files are located. The matching language file for the plugin will be selected if available.
To translate glFusion or any plugins to a new language, start with the english.php or english_utf-8.php file as the base.
Please submit any translations to http://www.glfusion.org/filemgmt/submit.php so they can be included in future releases.