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 more or less 100% in a single subdistrict)
In short:
You don't see many languages in the middle around 50% because there is often a dominant language in a district, which causes bumps at the high end (representing the percentage of native speakers of the dominant language) but also a bump at the low end (representing the percentage of native speakers of the non-dominant languages)
A nice way to add information to that graph would be to make a stacked graph where you sub divide the bars and give different colours to the 1st most spoken language, the 2nd most spoken language, and the other languages. In that way you can see how the mirror image is created from on the right the dominant (most spoken) language. And on the left the rest.