"All the world's a stage," wrote William Shakespeare. But not all the players upon that great and turbulent stage speak the great Bard's native tongue. To be usable on a global scale, your software needs to communicate in many different languages. The menu labels, button strings, dialog messages, titlebar titles, and even command-line error messages must be settable to the user's choice of language. This is the topic of internationalization and localization. Because these words take a long time to say and write, they are often abbreviated by their first and last letters and the count of omitted letters, that is, I18N and L10N.
Java provides a Locale class to discover/control the internationalization settings. A default Locale is inherited from operating system runtime settings when Java starts up and can be used most of the time!
See also Java Internationalization by Andy Deitsch and David Czarnecki (O'Reilly).