I am not sure that fluency is directly related to thinking in another language.
I have used for several months learning Italian (mainly) and other languages. I have also used as an introduction to toki pona, tok pisin and others. I would not call myself fluent in any of these languages as I would have little hope of holding a conversation (without the use of a dictionary/translation tool) in any of them.
I do, however, observe things and try to say something about it (what it is, for instance) in a language other than English (my native language).
Example: while reading this thread, I was drinking a cup of tea. I looked at the cup and attempted to say "the beverage is tea" in Italian. I did not consciously think of the sentence in English first, only speaking it after I had done so in Italian first (I got it wrong by the way!) There have been times when I have done this with other languages, including toki pona. (Often, after I say it in English, and think about it, I will realise that I got it wrong, but that is all part of learning, right!?)
It is possible that I am thinking it in English first, though not directly conscious of the fact. Not sure.