The most rampant and fossilized mistake I've heard from Chinese EFL speakers/learners is "a Chinese" where 'Chinese' was used as a singular noun, for instance, "I am a Chinese".
I know the right saying is "I am Chinese" or "I am a Chinese person" where 'Chinese' is an adjective, or "We are Chinese" where 'Chinese' acts as a plural noun or an adjective.
But some equivalent nouns can be used as a singular noun, for example, "an American", "a European", "a German", etc.
And some others cannot as 'Chinese', like "an English" and "a Japanese" are also unidiomatic.
I am just wondering if there exists a grammatical rule that draws a line between such nouns? Or is this tradition due to some kind of cultural differences or historical reasons? Or just because all elites in English world have this expression habit?