In the 2011 Census, grouped under Bangla is the Hajong language, claimed as their mother tongue by only 71,792 speakers.
Now watch the video below, and tell me whether the language variety you refer to as Bangla matches the language in the video.
Maybe compare it to the Bangla video:
The Hajong people have not given up their language name. Every ten years, when the Census enumerators come around, these 71,792 people do not say they speak Bangla. According to Ethnologue, the language is in good health — is in vigorous use and there is a vibrant music and literature, and little girls are teaching their classmates how to speak Hajong —
But whatever they may do, Hajong cannot get the govt to record them as an independent language at all. In fact, the only reason why their language name gets up there at all is because they are more than 10,000 in population.
The Hajongs are a very interesting people, because it seems that this community was originally Tibeto-Burman language speakers who switched to an Eastern Indo-Aryan language. But the TB substratum remains discernible even today.
Hajongs know the difference between Bangla, Assamese and Hajong. They also have no problem speaking Hindi and English as well. But many Assamese, Bangla, Hindi and English speakers do not even know they exist.
All the Scheduled languages are just like Bangla. Hindi is of course the most extreme case, but luckily no one wants it to be the model for how other languages behave. Learn about the languages being vanished in your names.