Something like this is already in place, even for anybody!
… and if you are below 2000 reputation, you are even rewarded for it, which is an encouragement too.
Of course, this means that your edit will be placed in a queue, and it will have to be peer reviewed to be approved first – but if you are confident enough that you can fix bad English, I'm sure nobody will reject your edit if they saw you wanted to contribute. If those edits are rejected, feel free to bring up an issue here, on Meta.
I think a message such as the one you propose would be much too intrusive. The reasons are:
If you see bad/wrong English, you'll edit it anyway, regardless of the OP being Italian, Chinese, French, Klingon, or even British or U.S.-American. Just edit it!
Having a flag displayed next to every post would, in my opinion, encourage discrimination against users of certain countries. And while on Stack Exchange, everybody is nice to each other™, I still have a weird feeling about this.
Even if this message were optional, why would your posts need more edit attention than others? Of course, I'm happy if people are open to edits and say: "I'm glad you fixed my English!", or if they admit that they have difficulties with the language, but the whole point of a community-edited site (with no restrictions in this regard) is that anybody should edit anywhere if they feel like it.