These days, I have setup(*) a Discourse instance in German for a relative and her non-English-speaking community. Even though I chose German as default language in the new setup wizard, the default categories and topics have been created in English (despite having an existing translation, as far as I can see):
That makes sense. However, as far as I can see, the ./discourse-setup step (as explained here) will generate the app.ymland start Discourse in one step.
Maybe a solution for this would be to ask for the interface language within the ./discourse-setup dialog?