The Language module is responsible for dealing with the available languages on the site. Site builders can choose to install one or more languages from a wide selection. They can even create their own custom language if necessary. The installed languages can then be added to things such as entities and menu links in order to control their visibility, depending on the current language. Apart from the installed ones, Drupal 8 comes with two extra special languages as well--Not Specified and Not Applicable.
The modules also handles the contextual language selection based on various criteria, as well as provides a language switcher to change the current language of the site.