lingua franca vs interlanguage

lingua franca and interlanguage both are nouns.

Word	Noun	Adjective	Verb	Adverb
lingua franca	Yes	No	No	No
interlanguage	Yes	No	No	No

As nouns, lingua franca and interlanguage are synonyms defined as:

lingua franca and interlanguage: a common language used by speakers of different languages

Other synonyms of lingua franca include koine.

lingua franca (noun)	interlanguage (noun)
a common language used by speakers of different languages	a common language used by speakers of different languages