Alien and foreigner both refer to people staying in a country of which they are not a citizen. Foreigner is the more general and friendly term and sometimes carries connotations of an exotic guest.
Study the example given below.
Most foreigners want to see the Statue of Liberty and visit the Metropolitan Museum.
Foreigners have a hard time learning our language.
It is important for foreigners to respect local culture and traditions.
Alien is a more legal and unfriendly term, emphasizing that a person is not a native or naturalized citizen and may owe his allegiance to a foreign country.
Study the examples given below.
Enemy aliens were imprisoned during the war.
Homeless aliens who fled their own country have a tough time finding work.
Alien can also mean foreign or strange.
Apple trees are alien to Kerala.
If something is alien to your nature, it is not compatible with your nature.
Lying is alien to my nature.