Sesquilingualism

The transitional zone can be regarded as the dialect boundary, where the two different dialects meet but there is no significant gap in mutual intelligibility due to the sesquilingualism of the native speakers.

In Pilch's terminology, sesquilingualism ("mastery of one-and-a-half languages") occurs when "all members of a given society speak the inferior [sic] language, but only some of its members speak the prestige language, usually in a more or less imperfect manner as a 'foreign language.'"