According to Sapir-Whorf hypothesis, language is a cognitive mechanism defining our cognitional patterns and therefore defining our [social] realities. Their works, as many more Whorfianist works, are easily accessible online.
According to Daniel Everett, language is [by convention] a cultural phenomenon. Here is a site with his bibliography and some of Dan's publications.
According to Ruth Millikan, language is a form of biological communication and as such is a property of non-human living [social] species as well - although they don't have to be quite social. The truth, as usual, is relative and out there.