What were Bakhtin's main concerns regarding language and how does he treat language?
Bakhtin believed that language should be studied for its dynamic, real life, elements of speech and social relations. He did not think, as some structuralists did, that language is an abstract system because such thinking ignored language as a living, subjective, system of fluxing social relations.
Bakhtin used the term "heteroglossia" ("hetero" meaning "different" and glossa meaning languages) to describe how language is an ever changing complex of social dialects, class dialects, jargon, fads, and intertextual references. Bakhtin concluded that the novel is the best genre to showcase the heteroglossia of language because the novel includes more elements of everyday speech and thereby, more dialects, and a greater range of social, class, and cultural languages. Bakhtin believed that poetry can showcase heteroglossia but that its limitations to poetic diction limit its emphasis on different languages and different voices. Therefore, Bakhtin saw poetry as largely monologic and the novel as dialogic. In other words, he believed that the novel is the literary genre which had the most elements of these different manifestations of language: class, social, interpersonal, and political conflicts and relations.
Bakhtin looked at how language operates with and through politics, ideology, class conflicts, intertextuality, and historical contexts. In other words, every utterance, once spoken, does not stand alone; it becomes involved in a dialogue (dialogic) with other utterances:
The living utterance, having taken meaning and shape at a particular historical moment in a socially specific environment, cannot fail to brush up against thousands of living dialogic threads, woven by socio-ideological consciousness around the given object of utterance; it cannot fail to become an active participant in social dialogue. (From Discourse in the Novel)
Just as there are different class, societies, and cultures within a given country or state (who use the same language, English for example), that dialogue amongst those people is also stratified with those different overtones that come from different classes, social groups, and cultural backgrounds. Bakhtin argues that with poetry, we mostly hear the poet speak: a monologic voice. But in the novel, there is more opportunity to hear the characters speak. And in the novel, we have more chance to hear different voices ("languages") from doctors and lawyers to the poor to politicians and aristocrats.
Even a single word itself is not simply the signifier of a signified meaning. That word/meaning will mean something different because of who said it, the historical context in which it is used, and so on. Bakhtin studied language in its living form, as a dynamic interplay, acknowledging historical context and social differences.