#Wikipedia is written in a language that is known by its three letter ISO-639-3 code. All these languages do not have a two letter equivalent like English (en) or Dutch (nl) have. These languages have their own localisation. This includes localisation for the Wikipedia mobile application.
Many people of us want to use this localisation on their Android phone. This is not that straight forward; you have to install an app that allows you to use a user interface in stead of the Android user interface itself.
There is a problem. Android is not ready to support languages that are only known by their three language code. It is probably an oversight and possibly because all the "big" languages have two character equivalents. However there is no language bigger than the language you grew up with.
NB duh is the code for Dungra Bhil, a language from India.