That person would be called a missionary. The work would be called missions.
spanophile?
Treason.
That is called imperialism or colonization. It involves one country exerting political, economic, and cultural control over another country or region. Imperialism can also refer to the dominance of one religion over others within a society.
An undocumented immigrant
It is absolutely impossible to force someone to have a genuine fellowship with the true God ("follow a religion in truth") yet very probable to force someone to follow one of the world's system of beliefs and that is called "abuse."
Someone who comes from another country is typically referred to as a "foreigner" or "immigrant," depending on the context. A foreigner is generally anyone who is not a citizen of the country they are in, while an immigrant specifically refers to someone who moves to a new country with the intention of residing there permanently or for an extended period.
refugees
Someone who betrays their country is called a traitor
Changing from one religion to another is called the same for any of them: conversion.
xenophobia
When you leave a country it is called emigration.When you arrive in a country it is called immigration.
A person who comes from another country is called an immigrant.