@gerhard
To get started with my tests I set DISCOURSE_DEFAULT_LOCALE : de_formal and did ./launcher rebuild app. Then I replaced all text using the correspondig button.
On my test instance a login is required. When I log out I get the massage:
Ein Konto ist erforderlich. Bitte frage ein existierendes Mitglied nach einer Einladung oder melde dich an, um fortzufahren.
The corresponding text in administration is " login_required.welcome_message_invite_only". I double checked in transifex that there is no untranslated string with this text. but discourse (running with Deutsch(Sie)) is still showing the informal text.
Is it possible that the source language file in transifex is missing some text?