The valley speaks Sylheti. Whether Sylheti is a dialect of Bengali or a language in its own right is a genuine and unsettled question: it has an ISO 639-3 code of its own (syl) and most descriptive linguists treat it as a separate Indo-Aryan language, while many of its own speakers, and a body of scholarship, treat it as Bengali speech and use standard Bengali for writing. Both positions are held sincerely and by people from the valley. What is not in dispute is the linguistic distance — Sylheti deaspirates and spirantises where standard Bengali does not, so an initial p tends towards f and an aspirated k towards a fricative, and it has developed tone, which is unusual in Indo-Aryan. Reported mutual intelligibility with standard Bengali ranges from easy to poor depending on the register and the listener.
SylhetiIndo-Aryan, Eastern; ISO 639-3 syl
The spoken language of all three districts and of the Sylhet plain downstream, with the same phonology on both sides of the international line and no meaningful dialect break at it.
बोलने वाले: Around 11 million across the Surma–Barak lowland and its diaspora
Standard BengaliIndo-Aryan, Eastern
The written and educated register everywhere in the valley — school, newspaper, court and public speech — and the language whose official standing the 1961 movement was about.
Meitei (Manipuri)Tibeto-Burman
An associate official language in the three districts, written in Bengali script and increasingly in Meitei Mayek, spoken in villages settled from the Imphal valley.
बोलने वाले: About 1.26 lakh returned in Cachar, Karimganj and Hailakandi (2011 Census)
BishnupriyaIndo-Aryan
An Indo-Aryan language spoken by a community that lived in the Manipur valley and carries Meitei vocabulary and a Vaishnav repertoire with it. Its speakers and Meitei speakers are distinct communities and the distinction matters locally.
DimasaTibeto-Burman, Bodo-Garo
The language of the dynasty that ruled from Khaspur, now spoken mainly on the hill margins north of the valley rather than on the plain its court sat on.
Hmar and RongmeiTibeto-Burman, Kuki-Chin and Naga
Spoken along the southern and eastern hill approaches, particularly above Hailakandi and Lakhipur.
SadriIndo-Aryan, Bihari group
The link language of the tea-garden communities, carried here from the Chota Nagpur plateau in the nineteenth century and now a first language for many born in the estates.