[LudwigNagasena]

> If native speakers say something, then it's grammatically correct.

What if two non-native speakers say something and understand each other?

[layer8]

That’s a pidgin.

[ricardobeat]

Then they’re having a useful, but not grammatically correct, conversation.

[antonvs]

It may be grammatically correct in the dialect they're using.

There's plenty of mutually-intelligible English spoken outside of primarily English-speaking countries which native English speakers would consider ungrammatical, but that's only relative to their own dialect.