Solved it.
How?
Create an own translation config with a different name than "default_configuration".
Default wise, there is a /libs/translationcfg/default_configuration which you cannot change. But there is also a /conf/global/translationcfg/default_configuration - here we can adapt the configuration. But it doesn't matter what we define; it has no impact. Something we need to check with Adobe, if this is per design.
So we have created a new translation config in /conf/ with a different name than "default_configuration". Now our configuration works.