I really thought this kind of issue was fixed.
Is the site’s locale set to default English (US), while the user’s locale is set to German, am I correct?
Looking at the error, the composer object doesn’t exist. Let me know how it goes.
I made a quick fix for that: FIX: Ensures composer translation object exists by Arkshine · Pull Request #11 · Arkshine/discourse-quote-callouts · GitHub