I'm trilingual, and I think only in english.
yes they do, why couldnt they if they were fluent? I dream in both too!
Yes, if you can say something in the language then you can think it too. I'm trilingual and tend to think in the same language I am post frequently speaking in/am surrounded by.
I'm bilingual and tend to base the language I think about around either my mood or environment. If not then I sometimes do this odd thing where I alternate the languages back and forth in my mind so my mind has each word in a different language.
If you live in the country of your second language, you often tend to think mostly in your native language, but often mix it with words, phrases and grammar constructs of the second. Also if you spend the whole day talking in the second language you often just come home and think for the first hour or so in language #2 before switching back to language #1
It depends on my mood if I'm angry or annoyed I think purely in Dutch and if I'm tired I tend to blur the two and get really confused.