In about 1950 my big sister went to a school in Switzerland where they were not allowed to speak any language but French and she was there for 12 months. When she came back she often broke into French and when asked why said she now thought in French. So, Snec, are you now thinking in French/
A friend of mine has lived and worked in France for almost 20 years. He works as part of a missionary society and when he returns to Scotland to visit his parents he is usually asked to speak in our local church, on more than one occasion whilst speaking he has had to ask the congregation the name of something as he cannot remember the english for it. He says it takes him about a week to stop thinking in French and then translating into English. He told me his French friends say he has a Parisian accent I think he is quite proud of that.
When I go to Australia I find that within a day of stepping out of the aircraft I sound less like me and more like them. It's almost impossible not to do it, and I'm reassured to hear I'm not alone in my madness.
I have the same problem. I've been living France for the last 2 and a bit years and when I go back to the UK I confuse people in shops as my default has now become "Bon jour, avez vous..." C'est la vie.