What were Bakhtin's main concerns regarding language? How does he treat language?

Bakhtin viewed language as an ever-changing interaction. Language is not some rigid structure wherein the rules and meanings are set in stone. Rather, language is a fluid product of individuals interacting with one another. Bakhtin devoted a lot of thought to Dostoyevsky and the novel. In Dostoyevsky, he found what he called "polyphony" or "many voices." Although each character in a Dostoyevsky novel is singular, all characters are informed by all other characters. This is a very simplistic way of saying everyone affects everyone else. Bakhtin goes into much more depth in this manner. He focuses on dialogue and the novel in particular as a literary genre because the novel contains characters, different dialogues, contexts, description, and socio-historical elements.
Bakhtin uses the term "carnival" to describe the idea of voices and people interacting. This conveys the idea of many different voices, maybe even something chaotic. But this helps to describe the diversity of interacting voices and how they (voices), in the context of a society, create meaning.
Another Bakhtinian term to be familiar with is "heteroglossia." Similar to polyphony, this term means differences in a language. We see this in a country as diverse as America, but this exists in any social group and even in a single individual.
Bakhtin's view of language is that it is the product of social, cultural, and historical factors. Meaning comes from the many ways these things factor in but also from the many ways in which we speak (heteroglossia). Consider the ways a single person speaks and makes meaning in a typical day. One speaks a certain way to her mother, to her husband, to her boss, or to some other kind of authority figure. So, there are different ways of speaking in the individual, different voices in a social group, different factors shaping culture, and Bakhtin felt that the novel showcased these interactions better than any other literary genre.