How many subdistrict are there? It looks like in most districts, about 2 or 3 thousand one language is dominant with 80% or more of the people that have this as native language. (and high dominance seems to be more likely than little dominance)

This leaves only 20% for the other languages in a district, and that creates this **mirrored image**. A language is either spoken by many (scoring >80%), or (as a consequence) on the other side only by a few (scoring <20%). 

(Possibly there might be some bi-lingual speakers, but I assume that in most cases the native speakers of languages should add up to 100% in a single subdistrict)