I am multi-lingual: born Dutch, living in Spain and speaking English for most part of the working day.
In the past it was easy to choose the language to use: Dutch, unless the other person doesn't speak Dutch, in which case I spoke English. I also choose English as my preferred language for everything related with IT, as more software and documentation is available in English, and because of the horrible translations in the first Dutch operating systems.
When I moved to Spain, I only knew a little bit of Spanish, so I decided to dive into it, and convert my whole life to Spanish. I wrote my own personal notes in Spanish, my PCs had the Spanish version of Windows, my to-do lists and calendar were in Spanish, and little by little my Spanish improved.
Now that I have lived here for a little bit more than six years, doing everything in Spanish started to feel a little bit forced. A few months back I changed the language of my mobile phone to English, and suddenly I realized that it is indeed possible to fit the texts on the tiny screen. My phone instantly became easier to use.
Then I got a new laptop. Though initially and by habit installed in Spanish, I felt more at home when I changed it into English (fortunately, in Mac OS X the operating system language is a user preference, so I didn't need to reinstall). And since the tools I worked with are in English, I also enter the data (to-do items, appointments, etc.) in English.
For each aspect of my life, I choose whether it is best to do in Dutch, English or Spanish. For example, whenever I am writing on El Canasto, I prefer that everything is in Spanish in order not to lose any time translating. Therefore, my brainstorming, saved text fragments and Movable Type are in Spanish. I also found that even though I am fully trilingual, you can't beat 37 years of experience in a language. Deeper thinking and difficult tasks are thus easier to do in Dutch.
You see: each language has its place.