Once I have settled on a language I use with another multilingual, it is uncomfortable for me to switch to another one. Feels fake.and I thought: 'thank God, it's not just me then!'
Once I have established a relationship with someone, I find it extremely difficult to switch to another language. As I mentioned before, switching languages with bilingual colleagues is awkward but feasible as both languages tend to be used for different purposes. I used to work in a bilingual environment. Where I taught French, a lot of the staff spoke English and French. With some of those people, we spoke mostly one language and if we had to switch for whatever reason I always felt somewhat uneasy.
But the true test was in my family. Before LJ was born, hubby and I toyed with the idea of speaking French together as we figured LJ would become dominant in English quickly if we had stayed in the UK. We tried! Well, he tried! He would come home and ask me what I wanted for dinner in French, told me about what was on TV in French (even though it felt weird: talking about English TV in French). I just couldn't do it. I would answer single word sentences and nod. When we wanted to speak about less trivial issues, we always ended up switching. And more often than not, it was me who switched back to English.
Why? Well, I am not completely sure. There may be different reasons. We had established a relationship (over 10 years) in English and using French felt to me like we had to start again from scratch. Also, even though his French is/was good, I felt like I would be misunderstood at times. I felt it was giving me an unfair advantage in a way. Whereas in English, we were both non-native speakers, in French we were unequal.
I know some people do it and very successfully. I admire them! I wasn't able to do it and in a way I am glad we didn't as LJ is very dominant in French now that we are in Germany. Had we spoken French, she would hear no English.