(There’s no particularly consistency with this, it’s just what sounds “good” to American ears. We’re perfectly fine with “as a German” or “as a Lithuanian.”)
No they are not.
The Oxford English Dictionary, for example makes it quite clear re. 'French':
"With plural agreement, and frequently with 'the' French people regarded collectively ..."
It is explicitly telling you that "French" is a collective plural noun and hence cannot be used as a singular countable noun.
sounds casual but correct to me
I don't care if it "sounds ok to me".
If you're going to make statements like that to go against what I've written then at least come up with some viable citations to grammar literature.
Honestly, in all my years on this earth I have never, ever heard anybody in any English speaking country I've spent time in say "a French" "a American" "a British".
And that amounts to a lot of time surrounded by people speaking VERY "casual" English.
P.S. I said "an American" was ok if you re-read.. an NOT a
For example:
* German is countable: https://www.oxfordlearnersdictionaries.com/definition/englis... * French is uncountable: https://www.oxfordlearnersdictionaries.com/definition/englis... * American is countable: https://www.oxfordlearnersdictionaries.com/definition/englis... * Spanish is uncountable: https://www.oxfordlearnersdictionaries.com/definition/englis...
But your explanation about why it is correct is bullshit, has nothing to do with "an" vs "a", the English language is just inconsistent as fuck and some demonyms can be used like this and some can't.