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. For the main glFusion language file, you'll want to look at private/language/english_utf-8.php, for plugins, you'll want to look at private/plugins/<PLUGINNAME>/language/english_utf-8.php. You can get these files from the current release archive, or from the glFusion GitHub Project Page.
To discover language differences between release versions of glFusion, you can use the diff tool in the glFusion GitHub Project Page to compare a release version with a previous release version.
Please submit any translations to http://www.glfusion.org so they can be included in future releases. Additionally, you can also use the Translation forum to discuss questions or to collaborate with other users on translating glFusion.