Hi and welcome to NethServer Community,
you’re right, the language needs to be added to the service file. (see for example SOGo new features: configuration template and access/configure button - #33 by mrmarkuz)
Add a systemd override: (replace collabora1 with your collabora instance name)
runagent -m collabora1 systemctl --user edit collabora
Add the following between the comments and save the file:
### Editing /home/collabora1/.config/systemd/user/collabora.service.d/override.conf
### Anything between here and the comment below will become the new contents of the file
[Service]
ExecStart=
ExecStart=/usr/bin/podman run --conmon-pidfile %t/collabora.pid \
--cidfile %t/collabora.ctr-id --cgroups=no-conmon \
--replace -d --name collabora --cap-add MKNOD \
--network=slirp4netns \
--env aliasgroup2=https://${TRAEFIK_HOST}:443 \
--env username=admin \
--env-file=%E/state/password.env \
--env "extra_params=--o:ssl.enable=false --o:ssl.termination=true --o:security.capabilities=false" \
--env dictionaries="de_DE en_GB en_US es_ES fr_FR it nl pt_BR pt_PT ru sv_SE" \
--publish 127.0.0.1:${TCP_PORT}:9980 \
${CODE_IMAGE}
### Lines below this comment will be discarded
Restart the collabora service:
runagent -m collabora1 systemctl --user restart collabora
Now the Swedish language should be available in Collabora.