Let's say you want to translate the system to Spanish. The locale in the example below would be "es" and the language locale would be "es_ES".
"language_title" => "[Name of the language, e.g. Español]", "language_code" => "[locale]", "intl_locale" => "[language_locale]",
Once done, a language dropdown will appear in the right top of the system.
More information on adding and configuring languages, can be found here.