Old English vs Kentish

Old English and Kentish both are nouns.

Word	Noun	Adjective	Verb	Adverb
Old English	Yes	No	No	No
Kentish	Yes	No	No	No

As nouns, Kentish is a hyponym of Old English; that is, Kentish is a word with a more specific, narrower meaning than Old English:

Old English: English prior to about 1100
Kentish: a southern dialect of Old English, spoken in the Anglo-Saxon kingdom of Kent

Other hyponyms of Old English include West Saxon, Anglian, Jutish.

Old English (noun)	Kentish (noun)
English prior to about 1100	a southern dialect of Old English, spoken in the Anglo-Saxon kingdom of Kent
	a dialect of Middle English, originally spoken over the whole southeast part of England, including London and Essex