I just checked if there were any languagepack or translation packages but there are none. Digging deeper it seems like all available translations get installed by default. There’s an old thread, Translation, which pointed me to the existence of .po and .mo files but there’s only a German one (English probably being hard coded as the default language):
root@ucs-addc:~$ find /usr/share/univention-* -name \*po -or -name \*mo
/usr/share/univention-management-console/i18n/de/serveroverview.mo
/usr/share/univention-management-console/i18n/de/ucr.mo
/usr/share/univention-management-console/i18n/de/udm.mo
/usr/share/univention-management-console/i18n/de/umc-core.mo
/usr/share/univention-management-console/i18n/de/services.mo
/usr/share/univention-management-console/i18n/de/apps.mo
/usr/share/univention-management-console/i18n/de/quota.mo
/usr/share/univention-management-console/i18n/de/udm-saml.mo
/usr/share/univention-management-console/i18n/de/appcenter.mo
/usr/share/univention-management-console/i18n/de/setup.mo
/usr/share/univention-management-console/i18n/de/lib.mo
/usr/share/univention-management-console/i18n/de/top.mo
/usr/share/univention-management-console/i18n/de/ipchange.mo
/usr/share/univention-management-console/i18n/de/join.mo
/usr/share/univention-management-console/i18n/de/adtakeover.mo
/usr/share/univention-management-console/i18n/de/adconnector.mo
/usr/share/univention-management-console/i18n/de/sysinfo.mo
/usr/share/univention-management-console/i18n/de/reboot.mo
/usr/share/univention-management-console/i18n/de/mrtg.mo
/usr/share/univention-management-console/i18n/de/admindiary.mo
/usr/share/univention-management-console/i18n/de/updater.mo
/usr/share/univention-management-console/i18n/de/diagnostic.mo
/usr/share/univention-management-console/i18n/de/passwordreset.mo
/usr/share/univention-management-console-frontend/de.po
/usr/share/univention-management-console-login/de.po
/usr/share/univention-portal/de.po
/usr/share/univention-self-service/www/de.po
/usr/share/univention-server-overview/de.po
/usr/share/univention-system-setup/www/de.po
The thread mentioned some ongoing discussion. Maybe they have some results now, 8 years later.