The default setting is Language->"English", unless a language kit is installed, in which case the default will be the appropriate language (e.g. "Japanese", "German", "French").
The setting affects many aspects of the system, including where configuration files, text resource files, system resource files, and documentation files are located. Each location containing language-specific files will contain subdirectories named after the language they contain. For example, a directory called Japanese will contain Japanese text resources and documentation. It is possible to have files for several different languages installed at the same time, with the Language option selecting which language is active.
When the value of this option is changed, the new setting does not take effect until the front end is restarted. It is not possible to switch languages during a session.