It appears that quoting multilingual content is quirky due to user behaviour.
If the user’s language is Japanese, they will see posts in Japanese. If they quote a post, it will be in Japanese. Then the user may choose to reply in English instead. 
This is interesting behaviour as firstly the feature attempts to determine the post’s language. If the post’s language is English, AI translation will not attempt to translate the quote which is a different language.
Es ist so wenn ich meine Sprache auf deutsch einstelle, das alle Zitate hier in diesen Thread auf deutsch übersetzt werden nur wenn ich es auf die Orginal Sprach English einstelle haben die Zitate ihre ursprüngliche Sprache
Now I see the reason why it happened to me. I pressed the “original version” button and it was left like that. As the original thread keeps to be in English and now I saw the German comment, I thought the translation failed or something. But I understand German enough to respond. So I quoted the original.
Only today I realized I had the “original” turned on and switched it back! So it was partly a UX problem. Maybe a warning “You are quoting the original version, are you sure?” would help.
