Assume we concur that God knows I'm now in pain apart from any external behaviors I might demonstrate like grimacing or shouting "ouch!" Does that automatically make the private language argument of Ludwig Wittgenstein wrong, and dualism true? That's a logically possible scenario; however, the inference from the given premises is not a necessary one. It might also be the case that God knows whether I'm experiencing pain without observing my behavior, and yet the private language argument might still be true. We first have to clarify what is meant by "private language" since the terminology is ambiguous. In this regard, Wittgenstein writes:
"But could we also imagine a language in which a person could write down or give expression to his inner experiences - his feelings, moods, and the rest - for his private use? - Well can't we do so in our ordinary language? - But that is not what I mean. The individual words of this language are to refer to what can only be known to the person speaking; to his immediate private sensations. So another person cannot understand the language."
If the private language argument as stated above is correct, then substance dualism is defeated, even if an omniscient God exists--and I believe he does exist. It's not possible to have one's own denotations for pain (a private way of expressing pain that only the individual understands) or to have a language that is inherently non-public. Language by its very nature is a public venture; even idiolects depend on sociolects.