Nationalities as proper nouns and adjectives

[tywysogesgymraeg]

Why can some adjectives describing nationality be used as proper nouns and some can't?
e.g. "He is German"/"he is a German". But not "he is French"/"he is a French.
Others are Belgian, Czech, Croat, Hungarian...
But not Spanish, British, Irish.

Why?

You could also say 'He is a Frenchman' or 'He is an Irishman' but not Belgianman, Spanishman etc.

That's no help, is it? Complicated it further!

Some, as you say, work in two ways - He is a German, He is German, etc. Some don't - He is Spanish, He is a Spaniard. He is French, He is a Frenchman.

Don't know why, though!