Let me try to show you the importance of language and that its vector (speech or other) has a minimal impact by answering the following question:
Can we think without speech?
The verb "think" refers to any conscious psychic (ie. mindful, not supernatural) activity, meaning the manipulation of abstract concepts . "Speech" is the human symbolic articulated language. According to these definitions, saying that speech is a necessary condition to thought is like saying an aphasic person isn't capable of any conscious psychic activity, which is fundamentally wrong. But if you take "speech" as "language" — a semantical communication mean  — then there is an interesting debate.
Broca's area, a brain region responsible for producing language, is also the center of manual activities recognition , which suggests that language and tool manipulation emerged on the basis of a common mechanism: thought . At this stage of human evolution, Homo Erectus spoke a proto-language that didn't allow the expression of abstract concepts ; yet, as soon as language appeared, the first forms of art developed, proof of a great abstraction ability . A form of thought emerged from the use of language.
This effect can also be observed in the development of language in kids, that will induce a restructuration of their reasoning ability thus allowing to master more complex forms of abstraction . Moreover, transmission of knowledge happens through language — in its pedagogical function — which contributes to developing thought; without this contribution of language, transmission of knowledge between generations would be greatly diminished and thought would evolve this much less over time.
Finally, the use of words — ie. the use of language — allows to densify complex concepts into simple sentences. This allows to bypass the limitations of human short term memory span  and to reach higher levels of abstraction and reasoning, only allowed by the use of language. Language is a basic structure allowing a complex thought mechanism .
However, you can find some types of thought that aren't related to language: our "technical and instrumental" thought process , some highly abstract Asperger's abilities , some perception tasks in monkeys .
In the end, language frees the mind by allowing for higher levels of abstraction (even though it constrains the thought by its limited set of concepts).
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