interlanguage (n.)

late 13c., langage "words, what is said, conversation, talk," from Old French langage "speech, words, oratory; a tribe, people, nation" (12c.), from Vulgar Latin *linguaticum, from Latin lingua "tongue," also "speech, language," from PIE root *dnghu- "tongue."

The -u- is an Anglo-French insertion (see gu-); it was not originally pronounced. Meaning "manner of expression" (vulgar language, etc.) is from c. 1300. Meaning "a language," as English, French, Arabic, etc., is from c. 1300; Century Dictionary (1897) defines this as: "The whole body of uttered signs employed and understood by a given community as expressions of its thoughts; the aggregate of words, and of methods of their combination into sentences, used in a community for communication and record and for carrying on the processes of thought." Boutkan (2005) writes: "In general, language unity exists as long as the language is capable of carrying out common innovations, but this does not preclude profound differences among dialects."

In Middle English the word also was used of dialects:

Mercii, þat beeþ men of myddel Engelond[,] vnderstondeþ bettre þe side langages, norþerne and souþerne, þan norþerne and souþerne vnderstondeþ eiþer oþer. [Bartholomew Glanville, "De proprietatibus rerum," c. 1240, translated by John of Trevisa c. 1398]

In oþir inglis was it drawin, And turnid ic haue it til ur awin Language of the norþin lede, Þat can na noþir inglis rede. ["Cursor Mundi," early 14c.]

 Language barrier attested from 1885.