Hmm, interesting question.
We think in the language of words that we know - memories are compartmentalized in our minds using the vocabulary we primarily use. How would one think without any basis for this compartmentalization?
I don't know; sounds a little disturbing to me - what that might be like - from birth to be without any structure (blind, deaf and dumb). I'd think ones mental existence would be a mass of unstructured feelings, sensations and chaos. Maybe not... Yea, there are a good-lot of examples of folks' perspectives who've been limited in their ability to communicate and perceive (and even more troublesome ones where folks lose it; ala "Johnny got his Gun").
... unless somehow the human mind has the ability to structure itself, absent of any external stimuli, to self-define. I'm going to have toss out a huge "I don't know" on this one.
From reading Steven Pinker and a little Chomsky, it seems that our mental faculties have more predefined structure, including universal grammar than you suggest. You seemed to have assumed more of a Blank Slate.
The counter to "we think in the words we know" would be my reaction to the first charging beast that I had ever met. I would not need to reduce the experience to my known vocabulary to instinctively react. I might reflect on the experience from high up in the tree that I climbed to escape, "that was a large, hairy, loud, cat-like, angry-sounding beast". But the words would have taken so long to express - even in the language of thought - that I would not have the opportunity to think them.