speak vs slang

speak is not a noun while slang is a noun.

Word	Noun	Adjective	Verb	Adverb
speak	No	No	Yes	No
slang	Yes	No	Yes	No

As verbs, slang is a hyponym of speak; that is, slang is a word with a more specific, narrower meaning than speak:

Other hyponyms of speak include run on, smatter.

speak (noun)	slang (noun)
	a characteristic language of a particular group (as among thieves)
	informal language consisting of words and expressions that are not considered appropriate for formal occasions; often vituperative or vulgar

speak (verb)	slang (verb)
express in speech	abuse with coarse language
exchange thoughts; talk with	fool or hoax
use language	use slang or vulgar language
give a speech to
make a characteristic or natural sound