For my part I know that I don't strictly think in words, or pictures, or any other such representations. I'm sharply aware of this because I'm relatively slow putting thoughts into words; so when I have some thought, I can consciously hold that thought in my head for a moment before the corresponding words come to mind.
For instance, suppose I realize I'm out of milk. It'll come to mind that I should go to a grocery this afternoon to buy more milk, but this intention doesn't form as a complete sentence in my head. It may literally take a couple seconds for me to recall the vocabulary I'll need to express this thought in words, but I can continue thinking about my trip to the grocery -- which one I should go to, what else I need to pick up there, etc. -- before any words have even come to my mind. So ideas and their linguistic representations are very distinct to me, and sometimes I don't even bother with the latter at all, because (for whatever reason) forming the linguistic representation of a though imposes a significant overhead on my brain. (I'm also dyslexic, for what it's worth, and I often wonder of these issues are related.)
My hypothesis is that for people with better functioning language centers than mine, words may come to mind so easily that the delay between thought and linguistic representation is imperceptible to them, hence they "think in words". But for people like me those words might come late or not at all, but regardless, I do think all the same...
Especially when I'm thinking fast thoughts - just assembling some concepts in my mind to see if they fit - I switch to English(not my native language) because it's words are shorter.
I don't think it's a dyslexia thing - at least not necessarily. As I wrote below, I also don't think in words, but I don't have any language problems that I know of (at least, I understand 6-8 languages, learned to read extremely early, and spell better than almost anyone I know).
I'm a bad speaker because I think so abstractly. I'm already thinking of the following concepts and their relations as I'm speaking, which is why I tend to stutter and pause constantly. I often find myself thinking in English because it has some representations my language doesn't, but there are things I can't conveniently express in English either. Because of this, I've been laying the blueprints for an artificial language of my own that would follow my line of thought. Synesthesia helps.
For instance, suppose I realize I'm out of milk. It'll come to mind that I should go to a grocery this afternoon to buy more milk, but this intention doesn't form as a complete sentence in my head. It may literally take a couple seconds for me to recall the vocabulary I'll need to express this thought in words, but I can continue thinking about my trip to the grocery -- which one I should go to, what else I need to pick up there, etc. -- before any words have even come to my mind. So ideas and their linguistic representations are very distinct to me, and sometimes I don't even bother with the latter at all, because (for whatever reason) forming the linguistic representation of a though imposes a significant overhead on my brain. (I'm also dyslexic, for what it's worth, and I often wonder of these issues are related.)
My hypothesis is that for people with better functioning language centers than mine, words may come to mind so easily that the delay between thought and linguistic representation is imperceptible to them, hence they "think in words". But for people like me those words might come late or not at all, but regardless, I do think all the same...