A country picker and a formal document don't necessarily want the same name. "Netherlands" is a useful label in a dropdown. "Kingdom of the Netherlands" is the formal name. Someone viewing the country in Dutch may want "Nederland".
Instead of choosing one and making you reconstruct the others, /country/NL?deep=true includes all three on a paid plan. Here are the relevant fields:
{
"name": "Netherlands",
"name_local": "Nederland",
"deep": { "full_name": "Kingdom of the Netherlands" }
}
Use name for the usual English label. deep.full_name is the formal English form when you need it. name_local preserves the local name when it differs from name; it is null when absent or identical. Japan's is 日本.
These fields describe different forms of the same name. They aren't a translation service, and name_local doesn't change with the reader's browser language. A country can also use the same text for more than one field, as Japan does for name and deep.full_name.
For most interfaces, name is all you need. The local name stays in the basic response; paid detail adds the formal name so that a more specific display doesn't require a second naming dataset or a guess at the formal wording.