What's wrong when it comes to spoken language is usually when you make mistakes likes slip of the tongue, e.g.:
It's right if other people who speak your dialect (other people in your speech community) also say the same thing systematically. In the Japanese case, it's clear that the construction is correct in his dialect, since he's not alone in using that construction. What's wrong when it comes to spoken language is usually when you make mistakes likes slip of the tongue, e.g.: