***** NOT A TEACHER *****
I agree with you: people do, indeed, often judge you by your language.
I have read, for example, that in England (before World War II), a police officer could tell what class a citizen belonged to as soon as that citizen opened his or her mouth. The cop would then treat that citizen accordingly.
As you know, here in the States, there will be a presidential election in 2016. I do not know who will win (although I have an idea!), but I do know that no candidate who uses "don't" when she or he should use "doesn't" would have a chance of winning. *
And probably any candidate who said something like "Me and my wife are delighted to be here" would never get elected, either.
In a social setting, you are right: I have no right to correct someone's grammar -- especially in public.
But in private, that's another thing.
I think that a good friend who is interested in your welfare should, in fact, gently point out certain mistakes.
In the early 1960's, when I was young, I would often say something like "Oh, he has already went to work." Someone gently and kindly reminded me that I should use the past participle "gone." I have noticed that quite a few people make the same mistake.
People should definitely not nitpick. And not in public. I guess that the only tactic to use against such people is to ignore them and keep your distance from them.
But I think that you should welcome sincere advice given in private.
It may not be fair, but we all judge people by how they dress and how they speak.
* We once had a president named Woodrow Wilson (World War I). I have read that in private, he regularly said something like "She don't like me" instead of "She doesn't like me." But in public, he remembered to use "doesn't." According to many books, many educated American and British people in the 19th century regularly used "don't" in such sentences until English teachers were able to stamp out that practice.
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