Castilian language vs Spanish

Castilian language and Spanish both are nouns.

Word	Noun	Adjective	Verb	Adverb
Castilian language	Yes	No	No	No
Spanish	Yes	Yes	No	No

As nouns, Spanish is a hypernym of Castilian language; that is, Spanish is a word with a broader meaning than Castilian language:

Castilian language: the Spanish language as spoken in Castile
Spanish: the Romance language spoken in most of Spain and the countries colonized by Spain

Other hypernyms of Castilian language include Spanish language.

Castilian language (noun)	Spanish (noun)
the Spanish language as spoken in Castile	the Romance language spoken in most of Spain and the countries colonized by Spain
	the people of Spain

Castilian language (adjective)	Spanish (adjective)
	of or relating to or characteristic of Spain or the people of Spain