The CountryDataAPI database has data in several languages:
In cases of specific data queries such as "All country data" and "All state data" requests, the lang variable will be a variable to distinguish countries with the same name between different languages.
Example:
If the country_name is "Spain" but the lang variable is es, the result will be empty. In this case you should search for the country_name "España"
In the case of a country whose name in different languages is the same, such as "Antarctica", using the lang variable will return only the country with that language