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Bwisi (also spelled Ibwisi, Mbwisi) is a language spoken mainly in the Kibangou District (Niari Region) of the Republic of Congo, next to the Gabon border, where it is also spoken by a minority. According to the Ethnologue, approximately 4,250 people speak the language today worldwide.
References
Bwisi at Ethnologue (18th ed., 2015) (subscription required)