I'm British, and spent 12 months as a placement student at CERN from 1995 to 1996. I believe that everyone had to have a reasonable competency in either English or French. They were then taught the other. I was already very good at French, hence I was placed in the top class for French, in which we spoke almost exclusively in French, learning some quite complex grammar. My fellow British student spoke no French, was placed in the bottom class, and after 12 months could get by. Some of his classmates, one Austrian in particular had no interest in learning French, and spent most lessons doodling high energy physics stuff.
My office consisted of three Spaniards and me. Our immediate manager was a French-speaking Belgian. His manager was Swedish. His manager was British (and was an old boy of the local rival school to me in Coventry, by huge coincidence). Thus, most conversation amongst us was in English.
In the final week of my time there, the Belgian explained something to the Spaniards and a visiting Parisian in French, then came to explain it to me in English. I told him I already heard. This was the first time he realised I was pretty much fluent in French, and swore/laughed for a while, when he realised that he'd never once thought to ask.
In the staff restaurants, most nationalities mixed a lot, and spoke either English or French, depending on the make-up of the group, with the preference tending towards English. The Spanish didn't mix so much, not for any other reason than keeping different working hours (MUCH longer lunch etc). Likewise for socialising outside of work. My "gang" was mostly British, Dutch, Belgian, French, Spanish, Austrian and German, and we mostly spoke English if addressing the whole group, but obviously the French and Belgians spoke to each other in French, and the Germans/Austrians used German.
For meetings and technical discussions, it was almost 100% English, unless it was known that everyone present was happier with another language.