au pair vs foreigner

au pair and foreigner both are nouns.

Word	Noun	Adjective	Verb	Adverb
au pair	Yes	No	No	No
foreigner	Yes	No	No	No

As nouns, foreigner is a hypernym of au pair; that is, foreigner is a word with a broader meaning than au pair:

au pair: a young foreigner who lives with a family in return for doing light housework
foreigner: a person who comes from a foreign country; someone who does not owe allegiance to your country

Other hypernyms of au pair include alien, noncitizen, outlander.

au pair (noun)	foreigner (noun)
a young foreigner who lives with a family in return for doing light housework	a person who comes from a foreign country; someone who does not owe allegiance to your country
	someone who is excluded from or is not a member of a group