un stranger es un extraño, alguien que tú no conoces. un foreigner es un extranjero, alguien ajeno al lugar

I fear both strangers and foreigners!

A foreigner can be a stranger and a stranger can be a foreigner.
A stranger is someone you don't know, a foreigner is someone from abroad. If you don't know a foreigner that person is a stranger as well as a foreigner. If you know that person, he/she is no longer a stranger but remains a foreigner. (And in the same way, a Spanish person you don't know is a stranger but not a foreigner.)

if you dont know the person, the person is stranger.
stranger can be in your country and you dont know the person.
foreigner is out of your country. you may know this person or not.