It is just a problem of social promise related with arbitrariness that we call
L2-learner's language interlanguage or approximate system and so on. But what is
L2-learner's language, that is to say, the definition of L2-learner's language is really
important. It is a starting point and basis to study L2-learner's language
pure-linguistically and applied-linguistically. Because, only if a definition of object
must be clear, we can study it properly. In this view, I critically reviewed
definitions on L2 learner's language in previous research, and tried to investigate
the nature of L2 learner's language. According to definitions of interlanguage in
previous research, it is neither L1 nor L2, it is just third language unlike L1 and
L2. And according to definition, interlanguage differs from individual to individual.
That is to say, it is indiosycratic. So any L2 learner cannot have the same
intelanguage. A basic concept to define a language is langue still. But we cannot
assume a language community that use the same interlanguage. So we also cannot
assume langue of interlanguage. Because langue is a rule system that people in a
social community share. Nevertheless, if we assume langue, that is a individual
langue applying just one person. On this thesis, I explained L2 language acquisition
as a process that L2 is gradually expanded and attached to L1. This explanation
is based on supposition that there is UG that L1 and L2 is sharing. There isn't any
reason that we suppose third area in L2 learner's language that is neither L1 nor
L2 during L2 acquisition, also there isn't clear evidence that the area exist. So
called, a language that is neither L1 nor L2 is, in fact, just the other side of L2.
That L2 is developing is a process for L2 to be complicate. So L2 that isn't fully complicated is also one side of L2 logically. But it is just defective and imperfect
L2 unlike native speaker's L2.