Wittgenstein was not arguing against the type of language you are talking about in his private language arguments. At length in various notebooks, Wittgenstein wrote about various possible people who speak a language without a social setting. He discusses Robinson Cruesoe in his notebooks and is completely ok with the idea of Robinson Cruesoe living by himself and using words, or coming up with his own language to describe how he goes about his daily tasks. What is important to Wittgenstein is for it to be a language, it must be in principle possible for you to observe him or interact with him, and get a sense of what he means. For example, maybe he leaves signs on the ground to help him remember where to find various natural resources, or to help him remember melodies of songs he writes, etc.
Wittgenstein is also not trying to put forward the argument that language has to be learned in a community. He discussed the possibility (again in his notebooks) of Martians who are born with an innate ability to speak. He doesn't believe that that is impossible or incoherent. How the person comes to develop language is not important. What he is arguing against is the possibility of a language which is necessarily private, ie not reducible in principle to any public notions. For example, someone who tries to privately ostensively define certain sensations he or she has. Or philosophers who claim that language is essentially private (ie the definition of pain is that private sensation you have, which leads to people saying things like ("I don't know if other people's pain is real, as I only know with certainty that my pain is real").
EDIT: Check out the Analytical commentaries on the Philosophical Investigations by PMS Hacker if you really want to delve deeper into some of exegesis of PI. The idea of Robinson Cruesoe, the Martians, etc, is discussed at length