The message you want to display in the method you have proposed will not do what you want.
A message without pointing out a problem is useless.
Simply saying "have you checked your spelling and grammar" without pointing out what specifically is wrong and where doesn't achieve much of anything. If you're not able to point out specific problems that need addressed, then the user isn't going to do anything about it. That would be similar to - actually, worse than - Microsoft Word just underlining the entire paragraph and stating "something isn't right here." Everyone is just going to click through it and ignore the message.
You're just annoying the users who don't have this problem.
Plenty of your user base does not have trouble with their English typing skills and don't have a lot, if any, spelling or grammar mistakes in their posts that this message is super useful to them. You're targeting a subset of your new users with a message that will annoy all of your new users.
Not all of our users are fluent in English.
It's a natural part of the world we live in. Lots of other languages exist out there with completely different grammar rules than English has, and not everyone is as proficient in English. What they typed into the box may very well look like correct English to them based on how they've learned their own language in the past, or based on the amount of English they've been taught so far. Their browser might not even tell them it's wrong if they have their browser language set to something other than English.
Asking them to review their grammar and spelling likely won't yield any results because they wouldn't even know what is wrong with their spelling or grammar until you told them. I get that no one really signed up here to teach English to people but if you want to interact with the rest of the world, sometimes you have to.
The best thing we could do to help you with this right now is modify the Ask Guidance that appears in the yellow info box at the right when asking a question. But even then, the advice needs to be better worded than just "Have you reviewed your spelling and grammar?" You may be interested in creating a meta FAQ that explains the standards of the site (proper grammar required, etc) and suggests online tools that these users could paste their post into for more detailed advice before posting. Then you could briefly mention quality standards and link to the post for more information and further help.