Alright, what about a child who sees cars everyday, but has never been told what a car is. He can still manipulate the car concept as easily as if he knew the word for it.
Or some people have very small vocabularies, simply because they have been exposed to a small number of words - does that affect their cognitive skills?
Abstract though and language may not be fundamental different things, I don't have enough information to say. But what I am saying in my post is that first you think in a concept, then you translate it into a word. You don't think in the word first. So you can think without needing to translate it into words. The relationship between the words and the concepts is of course strong, because we use words to vocalize the concepts we have.
Think about it - what is a word actually? It's just a series of sounds we create in the air. So the sound of a word is quite irrelevant to the concept behind it - the construction of the word is not relevant. Imagine we did not have mouths but instead we communicated by flashing a series of pictures in the air. The part of us that creates these pictures is just a sensory IO appliance like our eyes or mouth. Would that change us in any fundamental way? No - we would construct a similar grammar and communication method with pictures as we would with words.
What this implies is that the words themselves are unimportant, they are just representations of concepts we are dealing with. This representation can take many forms - but because we are used to vocalising our thoughts, when we do indeed think, we automatically prepare the things we are dealing with for vocalisation, leading to the illusion that words are actually involved.
What happens this is that this vocalisation becomes a sort of feedback loop for your thoughts - you think of concepts, convert them into words, then reabsorb the words to be reprocessed as if someone told you them. That is, they come back in as if you were listening to them from other people. You can observe this sometimes with people when they mutter to themselves. When we rehear our own thoughts, we can sometimes process them better (for reasons I don't know). But this step is optional, one can also do it without reprocessing, though this will fail with very abstract things that actually need concepts alien to us.