I don't think there is any scientific evidence about the question of whether we think only in language or not. But introspection indicates pretty clearly that we don't think in language necessarily. We also think in visual images, we think in terms of situations and events, and so on, and many times we can't even express in words what the content of our thinking is. And even if we are able to express it in words, it is a common experience to say something and then to recognize that it is not what we meant, that it is something else.