Many European research groups use English as their main language for communication. Also, outside academia many people in Western European countries and Scandinavia are proficient enough in English to communicate with. (Though in my personal experience, English proficiency in Germany is quite bad compared to e.g. The Netherlands or Denmark.)

The country I would be most hesitant about is France, they really don't like speaking English, and most French PhD theses are still written in French. This is really idiotic, all it does is slow down the dissemination of research; some acquaintances who did their PhD in France were looking hard for a non-French external examiner of their PhD, which is the easiest loophole if you want to write the thesis in English.

I believe it's a stereotype and it's certainly not the case in CS research labs. I did my PhD in a French research institution (INRIA). In my group, a majority of the people didn't speak French. And of course, PhD students could write and defend their thesis in English if they wanted to (I don't know if they had to find a non-French external examiner, but it's likely to be the case that some examiners are foreign anyway).

Because English is the lingua franca of most academic fields? If you are publishing in English conference proceedings and journals, isn't it reasonable to write your thesis in English as well?

(Note that English is the default language of theses in many European countries.)