Since Drupal 10 the documentation on updating the translations have changed: https://docs.civicrm.org/installation/en/latest/drupal/#i18n
What exactly do I have to do to deploy a language other than english for an existing installation? Do I need the cv core:install command at all or are there other ways to tell the database there is a new language?
Grab the localisation (l10n) files and unpack the l10n and sql subfolders into vendor/civicrm/civicrm-core/
You'll find the l10n files on the CiviCRM Download page or from https://download.civicrm.org/civicrm-VERSION-l10n.tar.gz where VERSION is a recent version of CiviCRM.
Example:
cd /var/www/html/drupal10/
wget https://download.civicrm.org/civicrm-5.27.2-l10n.tar.gz
tar -zxvf civicrm-5.27.2-l10n.tar.gz
cd civicrm/
cp -R l10n/ ../vendor/civicrm/civicrm-core/
cp -R sql/ ../vendor/civicrm/civicrm-core/
cd ..
rm -rf civicrm/
If you've done this correctly, you should end up with vendor/civicrm/civicrm-core/l10n and vendor/civicrm/civicrm-core/sql/
You can remove any languages you don't need by deleting them before copying the l10n and sql folders.
It looks like I simply don't need any database upgrade. Just downloading the latest language file from https://download.civicrm.org/civicrm-l10n-core/mo/de_DE/civicrm.mo and putting it into a l10n folder inside the Drupal private files directory does it for me. Not sure if the SQL files are required anymore.
The documentation ist a bit unclear to me about translation upgrading. Any clearification is welcome.