This answer really depends on whom you are speaking to. Some multilingual people dream in different languages and this depends on who they are interacting with within their dreams. For example, if in the dream they are talking to their French mother they would reply in French, however when speaking to their English teacher they speak in English. Other multilingual people, in their dreams, speak in any language they know without a particular distinction. Some people just speak 1 language all the way through. Anyways, I’m sure there are even more variances that we are not aware of!