The packages are contained in the standard Open-Xchange installation repositories.
For a complete list of currently available languages, have a look at Available_Translations.
Use the package manager from your preferred distribution to search for packages with names
Log into UDM and search for language packages using pattern open-xchange*lang*
Important: Currently the additionally installed languages are only available within Open-Xchange Groupware and cannot be set within Univention Directory Manager.