An ambassador
In general terms a professional who does this is called a "diplomat".
The person who writes on behalf of some one else is called a biographer.
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.
A person who comes from another country is called an immigrant.
A person who settles in a new country is called an immigrant.
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.
traitor is a person who is disloyal to hisown country.
A person from another country is typically called a foreigner or an international person.
A person who leaves a country to go live permanently in another is called an emmigrant,
exile
The best way to sign a letter for another peron is 'on behalf of'.