Cogito ergo sum (I think, therefore I am)
What do you mean by 'I"? And are the two uses in the above quotation really equivalent?
I suppose "I" is a mind (or person?) described by the phrase "it me". There are a lot of notable things the statement doesn't claim. It doesn't claim that that the subject is human (or even capable of speech or any form of communication). It doesn't claim that other minds exist (or are truly apart), leading to countless solipsists and omnisoul believers.
It is dependent on the subject actually thinking though. A fictional character can speak the phrase, and be described as a great thinker, but they do not actually think. (Lots of things exist without thinking, but fictional characters do not. Characters who are thought to be fictional might exist, though, in which case they are not truly fictional)
Still, I thought (or did I?) of a less controversial version to satisfy the most pedantic solipsist:
You think, therefore you are.
A thought happens, other than the seductions of language* how and why is an "I" imputed, and what exactily is "it"? Even if the "I" can be sufficiently characterised and supported as necessary**, is that the same thing as an ongoing exisitential coherence, since the thought that gives rise (possibly...) to its neccesity is pretty much instantaneous?
* The scope is pre-language, pre-communication, as you have noted.
** I'm not sure it can, not everything has to adhere to a subject; rather this "I" is a high level concept/organisation which imports a shit ton of stuff, exactly what is not wanted in such a fundamental statement.
(There are other solid crticisms of the historical phrase, for example that it sets up or presupposes a rigourous mind-body dualism which - all the King's horses and all the King's men, couldn't put Humpty together again... shit wait - which Descartes has trouble explaining the interaction of, see in particular the correspondence with Princess Elizabeth of Bohemia.)