For a multilingual person who speaks many fluently, what language would he think or dream in?

It varies. I know of one person from the USA who realized how thoroughly he had immersed himself in Latin America (where he was working) when he started dreaming in Spanish.

I knew one young girl, fluent in four languages and picking up a fifth, who dreamed in the language spoken at home.

It depends on what language(s) the person was speaking before they went off to dreamland. Sometimes a mixture of languages would occur within the same dream. It does not matter since this person UNDERSTANDS all of them.

The person would think in English if the current project is in English, they would speak Spanish or French or Chinese or Russian, etc... if the other projects require the language that they are fluent in.

You can't tell me now that if the person doesn't know either Chinese or Vietnamese but were told to translate from Vietnamese into Chinese when the person only knows English and Russian.

The language he is most comfortable with and what he know to be his main language. I myself know English as a 3rd language and it has become my main language

I'm wouldn't call myself fluent, but I have dreamed in Chinese and German before.