My First language is Portuguese, but having lived 9 years in the UK English has become my most dominant language where I dream and think with, the majority of times.

An interesting thing has happened to me twice already though, where I suddenly decided to speak Portuguese in a dream, one lucid the other non-lucid. Both times the DC I was with did not understand what I said and asked me to explain myself in English where they finally understood. I find this extremely bizarre.