If one pursues the internalist approach to language and the language faculty, one naturally considers one's own linguistic intuitions as a main manifestation of one's object of inquiry, i.e., the language faculty.

So, it seems to me to follow: if a native speaker of Japanese (mainly) works on a language other than Japanese, s/he is not an internalist.

Being an internalist also has another consequence: when one puts forth hypotheses to account for a set of linguistic intuitions of one's own, one considers whether what is stated by those hypotheses (or perhaps more accurately, the empirical content/consequence of those hypotheses) indeed feels like what goes on in one's mind when one judges sentences. This is about psychological reality of hypotheses in a naive (but important) sense.

So, it seem to me to follow: if someone working on her/his own language puts forth hypotheses to account for her/his own intuitions, but is only concerned with how things "work," and is not concerned about the relation between one's hypotheses and what seems to go on in her/his mind, s/he is not an internalist, despite the fact that s/he works on her/his native language. I.e., working on one's native language does not by itself make one an internalist.