Foreign - are from a country other than the one that they are living in (for example I am from England but I am a teacher in Brazil - to my students I am a foreign teacher. Overseas - live in a different country from their students (for example I teach online at italki and most of my students are in Asia or Europe. Therefore I am an overseas teacher).

foreign is foreign so overseas are overseas ..........

You use overseas to describe things that involve or are in foreign countries, usually across a sea or an ocean. He has returned to South Africa from his long overseas trip. overseas trade figures. = foreign Overseas is also an adverb. If you're staying for more than three months or working overseas :=)