slang vs speak

slang is a noun but speak is not a noun.

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

As verbs, speak is a hypernym of slang; that is, speak is a word with a broader meaning than slang:

Other hypernyms of slang include talk.

slang (noun)	speak (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

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