monolingual vs doer

monolingual and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
monolingual	Yes	Yes	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of monolingual; that is, doer is a word with a broader meaning than monolingual:

Other hypernyms of monolingual include actor, worker.

monolingual (noun)	doer (noun)
a person who knows only one language	a person who acts and gets things done

monolingual (adjective)	doer (adjective)
using or knowing only one language