Belarusian

The Belarusian language (Scientific: bielaruskaja mova) is the language of the Belarusian people and is spoken in Belarus and abroad, chiefly in Russia, Ukraine, Poland. It was also used to be called "Belorussian". It belongs to the group of the East Slavonic languages, and shares many grammatical and lexical features with other members of the group. Its predecessor was the Old Belarusian language (up to the 19th cent., conventionally).

In Belarus, the Belarusian language is declared as a "language spoken at home" by ~3,686,000 (36.7%) of inhabitants (1999). By the less strict criteria, ~6,984,000 (85.6%) of Belarusians declare it their "mother tongue". Other sources put down the "population of the language" as 6,715,000 in Belarus and 9,081,102 in all countries
Source - Wikipeadia.com

See also:

bbc.co.uk Country Profile - Belarus