The whole issue of language v/s dialect is a complex one; but I have adjusted to the one in this popular article [1]: if you speak a language and can enter into a conversation with a speaker of a variation on that language relatively easily, it qualifies as a dialect.
> A Mandarin-speaker can no more “adjust” to Cantonese than a Swede could “adjust” to German.
Most of the well-known Indian languages actually fail this test. Someone who has grown up speaking Hindi cannot reliably converse with someone in Bangla, Sambalpuri, or Tamil. If anything, the word "dialect" is overused regarding India – Bhojpuri [2] was often mentioned as a dialect of Hindi; but as a fluent Hindi speaker I cannot claim to understand or have a fluid conversation with someone speaking Bhojpuri. I am glad that it is at least sometimes referred to as a language now.
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[1] https://www.theatlantic.com/international/archive/2016/01/di...