I realized that the created translation package sets a wrong UCR variable in its debian/*postinst script. You need to set the following variable manually (or adapt the postinst script):
ucr set ucs/server/languages/fr_FR=Francais
I edited the po file of the UDM UMC module (it will change the description of the tiles shown on the UMC overview page):
msgid "Management of domain users"
msgstr "Gestion d'utilisateurs"
msgid "Management of user and computer groups in the domain"
msgstr "Gestion de groupes d'utilisateurs et d'ordinateurs dans un domaine"
I then followed the documentation for the build steps by executing (attention, in the manual, there is the suffix ".py" missing):
univention-ucs-translation-update-source-files.py -s ~/translation/ucs-4.1-1 -c fr
univention-ucs-translation-update-target-files.py -s ~/translation/ucs-4.1-1 -c fr
When logging in at UMC, I first chose in the upper right as language "Francais" (this is now visible as the wrong UCR variable had been set). After the login, my users category is now named "Utilisateurs" and user and group modules have the changes I applied.
I hope that helps.