Question: As an English speaking person, I think in English, and I assume French people think in French,etc. If someone was born deaf, and therefore could not learn how to speak a language, what language would they think in?

Philip Ratcliffe answered on 11 Nov 2014:

Now that’s an interesting one.
I actually speak Italian fluently (I’ve lived here over half my life) and certainly think in Italian most of the time (except right now since I’m writing in English).
The interesting experience I had early on was that as I started to think in Italian fairly soon but still didn’t have much of a vocabulary I actually often ended up feeling very stupid, not because I couldn’t say what I wanted to, but because I couldn’t even think it. That is, I couldn’t even put together logical arguments.
Your question is different though.
Now language is both written and spoken. Presumably, mentally one could use the written word in place of the spoken word and then of course the language would be that of the writing you were reading.
So, here’s a tougher question: deaf AND dumb. A very famous person, Helen Keller, due to an illness before she was two years’ old ended up this way but nevertheless invented her own form of sign language and then
with the help and patience of a special personal tutor learnt to read and write – in English. Presumably, before this she had very disconnected thoughts that were mainly a set of emotions and sensations.