Slav vs Croatian

Slav and Croatian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Slav	Yes	Yes	No	No
Croatian	Yes	Yes	No	No

As nouns, Croatian is a hyponym of Slav; that is, Croatian is a word with a more specific, narrower meaning than Slav:

Slav: any member of the people of eastern Europe or Asian Russia who speak a Slavonic language
Croatian: a member of the Slavic people living in Croatia

Other hyponyms of Slav include Serb, Serbian, Croat, Sorbian, Cossack.

Slav (noun)	Croatian (noun)
any member of the people of eastern Europe or Asian Russia who speak a Slavonic language	a member of the Slavic people living in Croatia

Slav (adjective)	Croatian (adjective)
speaking a Slavic language	of or relating to or characteristic of Croatia or its people or language