An interesting thing that happens in French-speaking research lab is that, since most of the scientific litrature we're exposed to is in English, the environment is constantly bilingual. Having to spend so much time reading in English, writing in English and studying communities where people express themselves in English, lots of people end up speaking in a homemade blend of English terms in French speech, with occasionally confusing results. I'm not always sure how much an outside observer would understand at first listen, but for those involved, it quickly becomes a natural way to talk. In fact, it's fascinating how difficult keeping to one language is.
I have no idea why, on a neurological level, mixing languages is so easy and keeping them separate is so hard (I should find time to rad more on the subject, one of these days), but I tend to think about it as a sort of switch that one has to develop over time. Here's the thing: when you learn another language, you very quickly come to realise that literal translations are usually impossible. People who have just started becoming fluent in one language or another are often delighted to tell the world about the many untranslatable concepts they've just discovered; but the truth is, concepts that have an exact correspondance in two languages are a tiny minority. Even perfectly straightforward words like 'table' or 'chair' don't mean exactly the same thing in French or in English, for instance, at least in their less common uses. So learning another language is not a business of learning how to say words from your language in another; it's a business of learning new concepts. And there's always room for new concepts. Theoretically, you could speak two languages at the same time with no more problems than having to make up an understandable grammar on the spot, and you'd have twice as many notions to express yourself with.
I suppose this is why it's so hard to keep languages separate in one's mind: new words don't get superimposed on words from your own language that mean the same thing, but rather, they come to fill in conceptual blanks you didn't even know existed. Keeping languages separate is extremely hard. It's like having to make an artificial partition in your brain, and developing a capacity to switch between one set of concepts and the other. And since sometimes, the first concept that will come to your mind will not come from the language you are currently expressing yourself in, you want to be able to find a corresponding word in the right language very quickly–an exercice that is not always easy, as some concepts are more often used in certain languages than in others. Needless to say, these abilities are mostly useful for translators, or for bilingual people expressing themselves in a strictly monolingual context. Others, like scientists, happily dispense from developing them.
The language switch is a rather fascinating device. For one thing, it tends to work better at some times than at others (when I'm tired, I sometimes forget which language I'm supposed to be speaking in, with occasionally hilarious results). It also works better between some languages than others. For example, my French-English switch is quite efficient, and so is my French-Spanish switch; but don't ever ask me to switch between English and Spanish. Other languages are not even fully compartimented: when I try to communicate with speakers of Slavic languages (using all twenty words I know in Russian, Czech and Ukrainian, plus a hefty dose of gestures and drawing), I have Japanese creeping in. When I'm really, really tired, I have words that come to my mind in a random fashion, and it can take me a good ten seconds to remember which language they're from. I wish it was a sign of an impressively multilingual brain, but it's actually not, unlike what the snobs who 'accidentally' let phrases in cool languages slip into their everyday speech appear to think: it's a sign that I haven't got far enough with some languages to be operational (because what use is a language when you can't isolate it?).
(It's very funny, by the way, how people who 'accidentally' mix up languages only ever do it with English, if they're French. No chance they'd 'accidentally' let a German phrase slip because the concept 'fits so much better'. And no, the Rule of Cool is not considered a scientific rule in linguistics)
Still, it's a bit of a shame to be speaking so much English, because Québec French is much more different from France French than I anticipated, and I wish I could learn to use it better. Instead I'm busy trying to keep tongues from jumbling up in my brain. At least I'm turning into a proper scientist…