We’re on the hosted instance plentymarkets.com with version Discourse 1.9.0.beta13 - https://github.com/discourse/discourse version e600fb79b3c3eb228f5ee7bf8674016964141bd0
Text customizations use the locale of the user that edits the text. So, you need to make sure that you use the German locale if you want to edit a German text. Currently three of them are in English.
locale | translation_key
--------+-----------------------------------------------
en | js.composer.reply_placeholder
en | login_required.welcome_message
en | system_messages.usage_tips.text_body_template