Whenever I hear someone using wrong English with my kids, I will rephrase that person's words, and then repeat it correctly to my kids. So far, my kids understand and is able to replicate my proper use of language.
I don't expect that person to correct him/herself (may be a force of habit after all). This way, I don't have to offend anyone, and the kids can still understand. And who knows, that person may eventually learn from you too!
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