Full Idea
'Mentalese' would be neither conventional nor a means of public communication so that even to call it a language is seriously misleading.
Gist of Idea
Mentalese isn't a language, because it isn't conventional, or a means of public communication
Source
E.J. Lowe (Locke on Human Understanding [1995], Ch.7)
Book Reference
Lowe,E.J.: 'Locke on Human Understanding' [Routledge 2004], p.146
A Reaction
It is, however, supposed to contain symbolic representations which are then used as tokens for computation, so it seems close to a language, if (for example) symbolic logic or mathematics were accepted as languages. But who understands it?