Honestly, I've never seen an option like this either. But also, you'd be able to help correct someone's grammar nearly anonymously.
The reason I created this thread is because of the thread I created here:
https://bitcointalksearch.org/topic/quality-of-english-621971The idea might be a bit more spelled out in that thread.
Is the scale of the problem really that large that it would require a forum modification to try and address it? I dunno. I think that we need to remember what we are and what the forum stands for.
This is a forum, on the WWW, accessible by people all over the world. We should be grateful that others, whose first language isn't English, try to learn English such that they can communicate on topics that interest them.
While there are regional boards, I think that the problem you outline is not a significant problem; it's annoying for native English speakers at worst. Just try to understand the topic, ask for clarity, and if you get none then simple walk away.
I think a better idea of this would be a "suggested edit" feature like on StackExchange-type websites. The author will receive a notification when an edit is suggested, can read over it, and then approve the suggestion. This is more powerful than the Grammar Nazi mode because the edits are permanent and used by default.
This isn't such a bad idea, but I think that it might be something to look at in the new forum software.