WordPress supports Localization using the gettext framework.
l10n global variable is used to store all the readers corresponding to the different languages supported. It is set in the function
load_textdomain($domain, $mofile) defined in [wordpress root folder]/wp-includes/wp-l10n.php. Following functions are defined which internally use this function:
load_default_textdomain(): loads the default textdomain
load_plugin_textdomain($domain, $path = 'wp-content/plugins'): loads text domain defined in the plugins
load_theme_textdomain($domain): loads text domain defined in the theme
Loading of the text domain involves looking up for corresponding mo file with the same name as the locale.
The default domain is loaded using the function
load_default_textdomain() in [wordpress root folder]/wp-settings.php.
Plugin or theme developers need not use this global variable directly. WordPress provides functions to translate text in localized language.
Back to full list of global variables.