It's funny how many people say there are some things they can only do in one language, whether it be talk to babies and animals, or swear (I enjoyed this article and the comments on it on Multilingual Living on the subject). I can really identify with that: to this day Thomas and I tend to speak German on the phone, for reasons that will become evident later in this post. But I have also experienced how these habits can change, sometimes unintentionally and often unnoticed through changing circumstances, but also sometimes as a result of a quite deliberate effort. Can I indulge in a bit of reminiscing and take you through some of our language chronologies to illustrate the point?
When Thomas and I met, we only spoke German. We were both living in Germany, I spoke German well enough to communicate everything I wanted to, and in any case I was there as part of my degree programme to improve my German.
This didn't change when we both moved away from Berlin. Over the next four years, each of us lived variously in the USA, in Russia, in the UK and in Germany, but never in the same country at the same time. We spent a LOT of time on the phone, always in German (hence I believe our tendency to revert to German when communicating via a piece of plastic without being able to see each other).
After this, there was a multilingually very boring period where we both lived in Germany (not in the same town, mark you, we couldn't do anything as obvious as that...) and spoke German ALL the time to everybody.
Then things started to get interesting again. Between us we gradually moved to the French-speaking area where we live now (and, Reader, I married him!) and both eventually took up work where we were mostly speaking English. Despite our emotional attachment having developed and become fully fledged in German, we gradually started to mix more and more English into our conversations without ever really noticing (except of course on the phone). And then the children came along, and it seemed fairly obvious to us to decide on the one-parent-one-language (OPOL) strategy.
All of a sudden, two things happened: firstly it actually took me some effort to make myself speak ONLY English with the children, so used was I to mixing languages in my home environment. And secondly, a significant proportion of my home communication was now taking place in English, as I inevitably spent more time with the babies than I did with Thomas (I was working part-time, so spending weekday time with them but without him). Then I got lazy: Thomas speaks fluent English, right, and is telling me stuff from his English-speaking work environment in English (why would he translate it for me???), so I more or less stopped speaking German to him (except, of course, on the phone!).
So much for it coming naturally to speak to babies in your mother tongue, and it being impossible to change the linguistic EQ of the language you fell in love in.
PS In a similar vein, you might like the interview with bilingualism researcher and adept Prof. Jean-Marc Dewaele, also on Multilingual Living. You might even want to take his survey on personality and code-switching. I did, and as with many such surveys, found it quite difficult to boil down my opinions to a yes-no answer on a subject I have rather more sophisticated ideas about. But definitely worth doing to support research in a very interesting area.