If you want more exact data just look up the relevant statistics.

I've travelled to many countries in Europe, and in my experience you survive with English from better to worse in the following order, but you survive in all of them: UK, Sweden, Finland, Norway, Austria, Germany, Slovenia, Estonia, France, Spain, Italy, Greece, Czech Republic, Hungary, Turkey, Croatia, Poland, Slovakia. Some relevant countries like Iceland, Denmark and Netherlands are missing. I interacted mostly with less educated people, so this info isn't necessarily relevant for university studies.

My comfort zone for living in a country for extended periods of time speaking only english would be somewhere before Slovenia or Estonia on that list. In France, Italy and Greece the problem was not only that fewer people spoke english, but even the people who knew english were really persistent in speaking their native language.

For example, in Germany people would start talking in German with me on the conversation?

It always takes less effort to speak your native language, so you should expect that.

Sometimes people start talking to you in a language you obviously don't understand. This has happened to me many times when I've asked a question in english. If this happens to me with a language I don't understand, I start talking Finnish back to them and smile. After a while they realize how ridiculous this is.