If you have the country name "Netherlands" and need the word "Dutch," a string function isn't going to get you very far. You need another piece of reference data.
That word is called a demonym, and /country includes it in Core alongside the country name. These are excerpts from two responses:
GET /country/nl
{ "name": "Netherlands", "demonym": "Dutch" }
GET /country/jp
{ "name": "Japan", "demonym": "Japanese" }
It's a small field, but it saves a caller from maintaining a list of exceptions or trying to construct English words from country names. If your interface needs that label, it can read demonym directly.
The country name still has its own job. You would show "Netherlands" in a country picker and use "Dutch" where the sentence calls for it. Keeping both values in the record means your application can choose the one it needs without guessing how to turn one into the other.