Anyway, I didn't mean to cause a fight. The mistake is a common one for Japanese speakers who speak English. I just wanted to point out that it is actually grammatically wrong in the context of the average speaker
It isn't grammatically wrong. Grammar has rules, people break or modify these rules in "common use" all the time. Grammar wins when you start talking technicalities and semantics. Common use wins when you are talking about "sounding natural".
I was taking issue with richvh's statement. I don't know any native speaker who would say "I'm a Japanese" sounds natural.
Richvh didn't say it sounds natural, either, he said it sounds weird. So, what exactly was your "issue"? So far as I can tell, you and he were in agreement on the matter, excepting in the case of whether or not it is "technically" correct. You put forth the challenge, and several of us have taken it up. You can't backpedal out of it now. So!
Technical information regards information from a reliable, widely recognized and endorsed authoritative source. For manufactured goods, it's a manufacturer's manual. For historical facts, it's an encyclopedia. For news, it's anywhere but FOX.
When one wishes to research the technical details of something, he consults the appropriate reference. In our case, it's a dictionary. Despite the existence of regional differences, we don't have "Webster's Southern Fried Dictionary" and "The Midwestern Dictionary" and "The Redneck's Giant Book of Words and Their Meanin's". We choose from a limited few that have been accepted as "true and correct".
Moving on to a hypothetical example:
Have you ever read a college thesis? The language used in a work of that magnitude is something you don't hear every day. Does that mean that the thousand page document that just scored someone a Master's degree in rocket science and a six figure salary is "technically wrong" just because the average Joe doesn't use the same words he does? I'd like to hear you tell him that.
If someone says, "I've lacerated the epidermis just millimeters from the surface of my ocular organ." Is that "technically" wrong? What if he's a doctor talking to his wife? Or to other doctors? Or a layperson talking to a doctor? He sounds weird to me, and I'd probably giggle and roll my eyes. I could tell him he sounds pretentious and he should have just said "I almost cut my eye!"
But, face it, he was "technically" correct, regardless of who he is or who his audience is. Any dictionary will confirm that his sentence was, in fact, grammatically sublime.
Same with the noun-status of the word "Japanese". It is a noun. Technically. By grammatical rules, then, one can say "I'm a Japanese" no matter how unusual it sounds.