foreign country vs res publica

foreign country and res publica both are nouns.

Word	Noun	Adjective	Verb	Adverb
foreign country	Yes	No	No	No
res publica	Yes	No	No	No

As nouns, res publica is a hypernym of foreign country; that is, res publica is a word with a broader meaning than foreign country:

foreign country: any state of which one is not a citizen
res publica: a politically organized body of people under a single government

Other hypernyms of foreign country include body politic, commonwealth, country, land, nation, state.

foreign country (noun)	res publica (noun)
any state of which one is not a citizen	a politically organized body of people under a single government