Japanese vs Nipponese

Japanese and Nipponese both are nouns.

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

As adjectives, Japanese and Nipponese are synonyms defined as:

Japanese and Nipponese: of or relating to or characteristic of Japan or its people or their culture or language

As nouns, Japanese and Nipponese are synonyms defined as:

Japanese (noun)	Nipponese (noun)
the language (usually considered to be Altaic) spoken by the Japanese	a native or inhabitant of Japan
a native or inhabitant of Japan

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