This happens mostly on SO.
From time to time, there are random (new) users posting random stuff that's not English. Those posts are often good candidates of the NAA flag. In case one of such post is in Chinese or Spanish, two languages other than English that I can read and write, I would post two comments under the post telling the author that Stack Overflow is English-only, one in the language the post is in, and the other in English, prefixed with "Translated:". Usually after some time, the English comment disappears on its own, which well indicates that it's removed by the community's flagging or a moderator (which doesn't matter, it means the community don't like it).
But in the meanwhile, I've been told, once or twice, not to post non-English comments. That's quite confusing since it's very often that when I post two, the English comment is deleted by the community.
So the question is, in such cases, should I post comments in the wrong language, or English, or both?
Sample comments I would post (almost verbatim, these are often written on-the-fly, so they differ by a little every time.):
欢迎来到Stack Overflow。请注意Stack Overflow是全英语的，所以在这里写问题或回答时请使用英语。
Translated: Welcome to Stack Overflow. Please note that Stack Overflow is an English-only site, so please write English when posting questions and answers.
The key is the first word of the English comment: "Translated:"
Besides, sometimes I promote local (Chinese) sites in the comment, and also translate it into English comment, which doesn't matter - the translated comment disappears while the Chinese comment persists until after the post is deleted.