In what language do scientists communicate with each other in European research institutions?

From my perspective of working in multiple European universities in different countries:

In a "closed" conversation setting, it will exclusively depend on the people involved in the conversation. The prime requirement is to use a language that every participant understands and speaks. A secondary concern is to use a language that people are particularly comfortable in. As a result, a meeting of two German researchers and a German student in a Swedish university will usually be held in German.

In an "open" conversation setting, people usually prefer to use a language that will allow other people to understand and join the conversation (usually English). Exceptions happen and might be considered as rude by some.

I am in Norway in mathematics. I use...

• Norwegian with the Scandinavians and Germans/Austrians. The people from Scandinavia also do this.
• Finnish with the Finn(s)
• English with the others.
• The Germans/Austrians speak German with each other. I have also heard a bit of French, I think, and certainly a bit of Spanish from other foreigners.
• Essentially, people speak the native language of some participant in the discussion when possible, and default to English otherwise.

Official announcements are almost always in Norwegian and often in English.

The situation was similar in Denmark: Danish was used with the people who could understand it (had been there for long or spoke a compatible language) and English otherwise.

Note that the non-academic people, especially older ones, might not be fluent in English.

I'm doing a postdoc in Europe, and my fellowship has me traveling around a lot. Also note that I do physics, not MCB, though I don't think there would be much difference.

My experience is that English is the official language of science, and all seminars, journal clubs, and talks (except outreach) are in English. Socializing will depend on the group. Also the language you hear the most at work has more to do with the nationalities represented in your group, not the local language. I heard a lot of Italian when I was visiting Switzerland.