Sociology of language focuses on the language

Language

Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

's effect on the society

Society

A society, or a human society, is a group of people related to each other through persistent relations, or a large social grouping sharing the same geographical or virtual territory, subject to the same political authority and dominant cultural expectations...

. It is closely related to the field of sociolinguistics

Sociolinguistics

Sociolinguistics is the descriptive study of the effect of any and all aspects of society, including cultural norms, expectations, and context, on the way language is used, and the effects of language use on society...

, which focuses on the effect of the society on the language.

A sociology of language would seek to understand the way that social dynamics are affected by individual and group language use. It would have to do with who is 'authorized' to use what language, with whom and under what conditions. It would have to do with how an individual or group identity is established by the language that they have available for them to use. It would seek to understand individual expression, one's (libidinal) investment in the linguistic tools that one has access to in order to bring oneself to other people.