Personally, I believe the mark of proficiency in a language is the point when you start thinking directly in it - when you stop explicitly translating to it from your native language.
That weird moment when you're not a native English speaker, lying in bed at night, pondering an open question while trying to fall asleep, then suddenly thinking to yourself: "Wait, why'm I thinking in English?"
For me, it seems to happen most often when I remember a certain word or phrase in English only, which causes my thought stream to switch languages.
Yes. At first it was weird, but then I got used to it.
The thing is, nowadays I read and write much more in English than in my native tongue, so there's an ever-increasing amount of things I default to thinking about in English. I also tend to switch languages on the fly in my thoughts, depending on which one "feels" better at any given moment.
The turning point where I started thinking in English increasingly was when I stopped watching TV shows and movies dubbed and went looking for the original version instead.
Curiously, I noticed that I still prefer to have subtitles on original versions. Not in my native tongue, but in English. Besides not missing some unintelligible dialogues, I found it reduces the feeling of cognitive load for me when watching a movie.
For me, it seems to happen most often when I remember a certain word or phrase in English only, which causes my thought stream to switch languages.