The Sylheti language
Sylheti [sometimes spelt Sylhetti, Siloṭi, or Syloti] is the main language of the Surma and Kushiara valleys of Sylhet Division in Bangladesh and the Barak valley region of Assam, India. It is spoken by over 10 million people, and is related both to Assamese and to the rural dialects of eastern Bengal, but with a high proportion of words derived from Persian and Arabic, and its own distinct grammar.
There are two things that make Sylheti an important language in its own right, and not a colloquial or 'peasant' corruption of Bengali as some have alleged. Firstly, Sylheti is the language of choice not just of the illiterate, but also of educated Sylhetis who can function well in Bengali when they choose to. Secondly, Sylheti is the only regional language related to Bengali which developed its own folk literature in its own script.