interlanguage

English

From inter- +‎ language. In the language acquisition sense introduced by Larry Selinker in 1972.

interlanguage (plural interlanguages)

A language generated by a student of a foreign language that incorporates aspects of their native language and the target language.
Trasyanka and Surzhyk are interlanguages: a Belarusian–Russian and a Ukrainian–Russian mixed language.
- 2011, Anna Trosborg, Interlanguage Pragmatics: Requests, Complaints, and Apologies, Walter de Gruyter, →ISBN, page 54:
  The learner's selection from his/her store of interlanguage rules is not haphazard but systematic and predictable, based as it is on his/her existing rule system in much the same way as the native speaker bases his/her speech on the internalized knowledge of the L1 system.
A lingua franca, a common language used by speakers of different languages
Synonyms: koine, lingua franca

Latin used to be the European interlanguage. Currently English widely serves this purpose.
- 2011 October 28, Adam Thirlwell, “The Joyful Side of Translation”, in The New York Times‎^[1], →ISSN:
  As [David] Bellos points out, those born as English speakers are now a minority of English speakers: most speak it as a second language. English is the world’s biggest interlanguage.
A pidgin or creole

mixed language

lingua franca — see lingua franca

pidgin — see pidgin

creole — see creole