Japanese vs Ryukyuan

Japanese and Ryukyuan both are nouns.

Word	Noun	Adjective	Verb	Adverb
Japanese	Yes	Yes	No	No
Ryukyuan	Yes	No	No	No

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

Japanese: the language (usually considered to be Altaic) spoken by the Japanese
Ryukyuan: the language (related to Japanese) that is spoken by the people of the Ryukyu Islands

Other hyponyms of Japanese include Ryukyuan language.

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

Japanese: a native or inhabitant of Japan
Ryukyuan: a member of the Japanese people living on the Ryukyu Islands southwest of Japan

Other hyponyms of Japanese include Jap, Nip, geisha, geisha girl, shogun.

Japanese (noun)	Ryukyuan (noun)
the language (usually considered to be Altaic) spoken by the Japanese	the language (related to Japanese) that is spoken by the people of the Ryukyu Islands
a native or inhabitant of Japan	a member of the Japanese people living on the Ryukyu Islands southwest of Japan

Japanese (adjective)	Ryukyuan (adjective)
of or relating to or characteristic of Japan or its people or their culture or language