Well, my goal was not “the countries”, but to learn “how to make a custom Id”. I could change the question for example to say:
“I have a model with Hotels with id, name and description, and I need the id to not be autonumeric but to be x or y text that I generate in my model OUTSIDE the Strapi thing. The description needs to be localized.”.
The question is more about Strapi model/configuration than “the countries content”.
PD: Thanks for the tip, though, about the countries API 
PD2: Maybe I should change the title to “How to create a non-localized custom ID field + a localized text field” but IDK how to change the title…