For Bakhtin, "language" is made up of the diverse (heteroglossia) utterances of speaking subjects. Bakhtin was interested in the many discourses and dialects of language that reflect the many individual, social, and political realities of the speaking world. Therefore, his use of the term "language" was based on a cultural analysis. For him, language was a diverse array of articulations.
Saussure, in his Course in General Linguistics, articulates two objects of linguistic study: la langue and parole. La langue, in Saussure's usage, is the abstract structure of all language. La parole (speech) is made up of the concrete utterances of speaking subjects. This is Bakhtin's focus when he writes about the many discourses and dialects that form the diverse socio-linguistic "heteroglot" in everyday life and in novels.
Saussure, on the other hand, focuses his study on "la langue." Saussure's study of language was intended to be scientific, a science of language that would be called semiology.
Both Saussure and Bakhtin were focused on language but they had different approaches to the method of study because they had different agendas. Simply put, Saussure studied abstract structure: a single unified system. Bakhtin studied the diversity of speech and language as reflected by the diversity of individuals and society. Saussure was interested in the abstract structure of all languages, making his style analytical. Bakhtin was interested in the actual usages of speech in life and literature. This puts Bakhtin's style more in the field of the Continental theorists who acknowledged the role of history and culture in all theoretical approaches, even to linguistics. This is why Bakhtin, in Rabelais and His Work, was interested in the folk humor and "carnivalesque" aspects of Rabelais' work:
Rabelais is unique. In his creative world, the inner oneness of all the heterogenous elements emerges with extraordinary clarity. His work is an encyclopedia of folk culture.
To be clear, Saussure was articulating an objective study of language as signification; the abstract structure and function of all languages. Bakhtin was articulating the various ways language is employed in speech and writing. As a result, Bakhtin gravitated more toward the novel than poetry because there he found a greater diversity of discourses and dialects: more varied employments of languages. It was in what he called the "carnivalesque" atmosphere that there seemed to be a freedom in which these many varied utterances could engage in dialogue.