[embedding-shape]

> 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.

[japanoise]

I'm using the words 'grammatical' and 'ungrammatical' in a linguistic sense; human languages are subtle and fluid, and one doesn't have to be far along the sliding scale between 'doesn't speak a word' and 'well-educated native speaker' to be understood. We speak of 'broken' English when somebody is able to be understood but hasn't fully grasped the language yet; using demonyms incorrectly is a subtler flavor of the same thing. For example 'no come here' -> 'no entering' -> 'no entry'