When I was a kid our family went on vacation to the US. Everyone kept asking if I was Dutch, which I thought was German (Deutsch).
So I kept correcting them, saying I was Netherlandish :)
I can tell you that this is called demonym, but I don't know the answer to your question... The Wikipedia page has a long list of suffixes, but no rules:
https://en.m.wikipedia.org/wiki/Demonym
The answer is that many languages import their demonyms from different foreign languages. The reason for the inconsistencies is the different, unrelated sources for words.
Oh there's plenty of rules, and if you follow them you'll be wrong because each rule has 20 exceptions you have to memorize because English isn't a language, it's several languages in a trench coat.
Thailand comes from adding the Germanic -land suffix to the demonym Thai, a common pattern for non-Indo-European places. There’s also Swaziland and Somaliland (though there is also a Somalia).
Meanwhile there is no specific demonym for people from the united states, you can say american buy that would also include every other north and south american country
Literally nobody who isn't a Latin American with a chip on their shoulder has a problem distinguishing Americans from "people who live on either north or south america"
Nobody calls Mexicans or Canadians Americans. Nobody calls Brazilians or Peruvians Americans. They maybe North Americans and South Americans but American means someone from the United States. The Canadians and Mexicans I know would be offended if I called them American.
People outside the US all assume "American" means US. Nobody thinks there's even a small chance you are referring to anything else. If you want to refer to South Americans you say "South Americans"
What the fuck are you talking about? The Chinaman is not the issue here, Dude! I’m talking about drawing a line in the sand, Dude. Across this line, you do not. Also, Dude, “Chinaman” is not the preferred nomenclature. “Asian-American” please.
Ray Belli is amazing and I've failed to learn so many things from his podcast because as soon as he starts speaking my mind wanders. It's like the audio version of reading the same paragraph four times because my brain decides to think about something else while my eyes move across the page
People from Indiana are called hoosiers - this, like many things in English, doesn't have a hard and fast rule... the sounds at the end of the word certainly impact it, but there are exceptions. Just ask a Peruvian.
FYI, there's a little debate over this in the English language, but many would say that the proper demonyms are Afghan for the Pashtun ethnic group, and Afghanistani (or rarely Afghanese) for people from Afghanistan regardless of ethnicity.
Afghani is their currency.
I believe it comes from a discrepancy between the Persian and Pashto languages. Afghani being the correct term in Persian, and Afghan being the term in Pashto.
Afghani is pretty widely used in English, and even appears in some dictionaries, but many argue that it's not correct.
So a person is an Afghan, they eat Afghan food, wear Afghan clothing, have Afghan customs, and their currency is the Afghan Afghani (in case some other country ever adopts a currency called the Afghani and you need to differentiate between them)