An ambassador
In general terms a professional who does this is called a "diplomat".
A chief diplomat is a government official responsible for managing a country's foreign relations and negotiations with other nations. This role involves representing the country in international meetings, engaging in diplomacy to resolve conflicts, and promoting the country's interests abroad.
The person who writes on behalf of some one else is called a biographer.
A person who acts may be called an actor or an actress. A person who acts on behalf of another is an agent.
traitor is a person who is disloyal to hisown country.
A person who leaves a country to go live permanently in another is called an emmigrant,
The best way to sign a letter for another peron is 'on behalf of'.
A person who is disloyal to their own country is typically called a traitor or a defector.
exile
Patriotism is the term that describes someone who loves their country fervently and demonstrates strong support for it. This individual is enthusiastic about their country's well-being and takes pride in its values, culture, and traditions.
a person disloyal to his own country
They are called immigrants, a person that comes to a country where they were not born, to settle there.