For this feature you need to have the rights of an administrator.

All languages ​​can be found under System -> General settings -> Language settings

There you will find the following page:

This page can be different as languages will be added.

The languages ​​that are checked are still available in the dropdown menu of the available languages:

If you want to add a language, you place a check mark next to the desired language and it will be available in the aforementioned dropdown menu.