3 invocations of hook_multilingual_settings_changed()

locale_add_language in includes/locale.inc: API function to add a language.
locale_languages_delete_form_submit in modules/locale/locale.admin.inc: Process language deletion submissions.
locale_languages_overview_form_submit in modules/locale/locale.admin.inc: Process language overview form submissions, updating existing languages.

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.