The flag is definitely helpful. But it requires that I compare the original post and the translation I see.
When I don’t do that, an indicator that I was looking at the German version when I flagged as off-topic would still be helpful. Especially in cases where the original isn’t in English, so I cannot tell without research if the translation or the original is the problem.
Does the custom flag affect trust levels? It would be unfair to punish users for mistakes in translations they didn’t write.
Edit:
I tried it right away