there is a time and place for everything. "Les règles de bienséance" matter more to me than the safekeeping of the exactness of English grammar, which as others have been keen to point out is hardly as strict as you seem to imply.

And no, no French person likes to receive corrections in grammar. Giving lectures on proper english grammar/pronounciation is generally a mark of (classist) pedantry since speaking proper english is generally the preserve of those lucky few that have had the opportunity of spending time in the Anglosphere, a tiny minority of the french population in fact, who are always eager to put their one upmanship on display, in a very crude, almost vulgar fashion.

I have been travelling through Japan for the past week, the grammatical and orthographical error would likely give you a nosebleed. Meanwhile, I just smile and move on, I got the meaning, it is what matters. Same for the OP.

I'm not sure how happy they actually are about it though. I think most people have a bit of Stockholm-syndrome relationship with it, the highest tier of argument refutation in France might honestly be grammar-based :P

(And it did motivate me to go abroad.)

> but wouldn't say 'a Thai', 'a Swedish', 'a Sudanese'

You also don't say 'a Japanese' but that is an extremely common error with Japanese English speakers when they are first learning.

I am looking for a citation, but I seem to recall the casual rule of thumb is something to do with the ending of the nationality (so '-ish', '-ese','-ch' etc. you can't put 'a' in front). I think the more formal explanation likely centers around rules relating to indefinite articles.

> but it is ungrammatical and a clear marker that the speaker is not native

You mean a native speaker might be ungrammatical when using their non-native language? That makes sense to me.

> Spain has 'Spaniard'.

Even so, you'll hear a ton of native Spanish people saying "As a Spanish person" or "As person from Spain" instead of simply "As a Spaniard", I'm not sure this is very surprising. If anything, that mistake makes it more likely they're a native than not, in the case of Spain, as the level of English outside of metropolitan areas is lacking at best, compared to other European countries.

There are some suffixed with "-i" which sound natural to my (American) ears too: "an Israeli", "a Somali", "a Pakistani", "an Omani", etc.

As a Welshman, I’d say North/South Walian are more common among the populace!

When speaking English, the French side of my family refers to themselves like that often, however, they're from Bretagne, so exactly how French they are is up for debate.

Demonyms don’t use the same rules as countable nouns. Both “French” and “British” are acceptable demonyms, they’re just not particularly idiomatic in American English (which likes to overcorrect with “person” like you’ve noted).

(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.”)

a French; an American; a Brit, or a British

sounds casual but correct to me

> "A Brazilian" or "a Spaniard" are acceptable.

Well, context is important on the Brazilian front. ;)

"I had a brazilian at my house" could have other connotations.

> or Frenchman if you're old and inconsiderate.

Or talking about a man that is French. Neither of which would be considered 'old', or 'inconsiderate".