The term interlanguage was coined by Selinker in 1972 in response to a highly influential paper, "The significance of learner errors," by Pitt Corder, published in 1967. 

Corder, an early applied linguist, showed that learner errors were not random, but followed common patterns.  This insight led him to describe learner language as "systematic," a "built-in syllabus," and paved the way for interlanguage research.

Read the original article:

Corder, S.P. (1967). The significance of learner errors. International Review of Applied Linguistics, 5, 161-169. PDF