What does learner language mean? Is it the L1 of any speaker?

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' Learner language' simply refers to language produced by L2 learners, as the name implies. Indeed, it is a term that can be applied regardless of L1 (or L2 for that matter). Thus, for example, a German learner of L2 Spanish, a Spanish learner of L2 French, and a French learner of L2 German all could be said to produce 'learner language'. The term emphasizes the fact that that set of language was not produced by a native speaker, hence it will generally be non-targetlike in some way.