Langue d'oil vs French language

Langue d'oil and French language both are nouns.

Word	Noun	Adjective	Verb	Adverb
Langue d'oil	Yes	No	No	No
French language	Yes	No	No	No

As nouns, French language is a hypernym of Langue d'oil; that is, French language is a word with a broader meaning than Langue d'oil:

Langue d'oil: medieval provincial dialects of French spoken in central and northern France
French language: the Romance language spoken in France and in countries colonized by France

Other hypernyms of Langue d'oil include French.

Langue d'oil (noun)	French language (noun)
medieval provincial dialects of French spoken in central and northern France	the Romance language spoken in France and in countries colonized by France