The bug was a little bit different from what you described:
- It always used the
default_locale
for the system message. - It used the flagger’s locale for the flag reason.
So, in your example, the system locale was set to English and the locale of the user who flagged a post was set to German.
https://github.com/discourse/discourse/commit/badad80cdf4489dd687a723647091c0b6b273ae6