I seem to remember reading that Wittgenstein wrote about 'private
languages' and argued that they were impossible. See the references in
http://pears2.lib.ohio-state.edu/FULLTEXT/JR-PHIL/thurman.htm (in which
Robert Thurman, father of the more famous Uma, compares Wittgenstein's
position on this to a mediaeval Indian philosopher called Candrakiirti
--- over my head).
Martin