Tshiluba
The constitution does not specify which variation of Tshiluba is a national language. There are two major variations of Tshiluba: Luba-Kasai, spoken in the East Kasai Region (Luba people) and Luba-Lulua, in the West Kasai Region (Bena Lulua people). Luba-Kasai seems to be the language used by the administration.
Read more about this topic: Languages Of The Democratic Republic Of The Congo