What Language Do You Think In?

A question my daughter asked me:

If someone is deaf and blind, what language do they think in?

My father was a linguist. He was fluent in several dialects of Chinese, plus Japanese, Korean, and German. He also knew some Spanish, French, Russian, and a few other languages that I'm forgetting. I joke that I know more languages than he did (but they're computer languages). He used to tell me was that one of the hallmarks of fluency in a language was thinking in that language while speaking it (instead of thinking in one's native language and translating).

Sad to say, although I studied both Spanish and Russian, I never got even close to fluent in either language. But, occasionally, if I get asked a question in one of the languages, I will find myself responding in the language without first thinking of the response in English.