Native English speaker, with casually acquired bits and pieces of many languages floating around in her head and several years of formal German study. It’s a little rusty now and at my best I was certainly not fluent, but I’ve always been told that I speak German pretty well…for an American, at least.
Normally I think in English, but within a few months of beginning to study German I found that I could think in German when I wanted to, and sometimes I did so without even trying. My limited vocabulary means that there are some subjects I can’t think too deeply on in German, but if I get myself going I can think about practically anything on a basic level using the words I do know. Sometimes I do this just to amuse myself, or in hopes of preventing my German skills from degenerating too much.
Very rarely I will unexpectedly have a German thought among my normal English ones, usually a thought involving a concept that for reasons of grammar or vocabulary is more easily or elegantly expressed in German than in English.