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 forementioned dropdown menu.