The [Account Settings](https://phab.urz.uni-heidelberg.de/settings/panel/account/) panel allows a user to change their preferred UI language. The default is "Server Default: English (US)". However, I am unable to find a config option to set this server default.
I had a look in the [UI config](https://phab.urz.uni-heidelberg.de/config/group/ui/) and the [Translations config](https://phab.urz.uni-heidelberg.de/config/group/translations/) as well as searching through [all options](https://phab.urz.uni-heidelberg.de/config/all/) for "translation" and "language".