During startup the system searches for all available languages. The system administrator can then decide which of those languages should be enabled. Enabled languages can then be selected by users in the personal settings or on the login screen.

To enable languages, go to Administration > System configuration > Languages > Active system languages

Select the languages you want to activate. Next to each language the translation status is displayed. It's no problem to activate a language that is not fully translated, the system will automatically fallback to the default language