The question of the language of thought has been addressed by philosophers for centuries. Socrates, in Plato's Theaetetus, argues that thinking is no more than silent speech. Here are his own words (Hamilton and Cairns, 1961):

Socrates: And do you accept my description of the process of thinking? Theaetetus: How do you describe it? Socrates: As a discourse that the mind carries on with itself about any subject it is considering. You must take this explanation as coming from an ignoramus; but I have a notion that,

when the mind is thinking, it is simply talking to itself, asking questions and answering them .... So I should describe thinking as a discourse, and judgment as a statement pronounced, not aloud to someone else, but silently to oneself.