My mother tongue is Russian, and I can't remember the last time I've been thinking in it for prolonged periods of time.

It's a beautiful language, but I have only been able to hold a few meaningful conversations using it throughout my life, most of which were between me and a Russian-Australian friend who spoke Russian and English as good as I did. Added is the fact that I haven't seen much meaningful content in Russian for a very long time - in fact, I can't remember any by now. That being said, I use Russian when I write drafts of my stories, because it is still difficult to write directly in English: I had no practice in it until later.

I knew enough English to speak it since I was... somewhere around ten to thirteen years (my memories of the time are fuzzy at best). I don't remember how it started, but from late teen years to now I've been mostly speaking English in my head. As I learn new languages - German, Norwegian - I notice myself slowly incorporating the constructions and the language itself into the thought process, for it allows for easier information translation and expression. Every language has their own simpler methods of expressing a certain idea - for example, where in Russian you have to repeat the verb to say "I do", English makes it so much simpler by using auxilary verbs in its stead - and incorporating those allow for a more meaningful conversations (aspects of a word matter).

By now, I believe it also has to do with the fact that I seek content mainly in English; I prefer the English Wikipedia for most topics, and I can recall only one Russian TV series that I enjoyed. It's both that English is everywhere and that I've chosen English over Russian that make the choice of internal dialogue language more ingrained. When I speak Russian aloud, I sound alien to the norm because I copy the style and the syntax from English without thinking about it.