Castilian vs Spanish language

Castilian and Spanish language both are nouns.

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

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

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

Other hypernyms of Castilian include Spanish.

Castilian (noun)	Spanish language (noun)
the Spanish language as spoken in Castile	the Romance language spoken in most of Spain and the countries colonized by Spain
a native or inhabitant of Castile