Why is it mostly a preferred option to use an adjective as a subject complement when it comes round to national identity whereas "American" can be both a noun and an adjective?
I am American
Why not: "I am an American".
Even more complicated is with a pattern like:
I'm Southern UK
why not: "I'm a Southern UK"?, "Southern" is an adjective and UK seems a noun but only at first glance.
It all gets clear when there are only adjectives for naming the aforesaid:
I'm French (Belgian).
And as for the Poles there are different parts of speech that can be used:
- I am a Pole (and it can't be changed to "I'm Pole)
and
- I am Polish (and it can't be changed to "I'm a Polish).