Well, almost all Germans have learned english in school from 1st grade.
So everyone will understand if you talk in simple words and slowly. Pronounced correctly, and clear it won´t be any problem. The more north you travel, the more english ist understood and spoken (though sometimes with a funny accent (i.e. in the netherlands)). In those countries the films are not synchronized, so they are used to the english language.
The more you go to the south, the less english ist spoken.
And some folks just don´t want to speak other than their own language.
I.e. I live near the french boarder, almost all french people living in that area do speak german, but when adressed in German, they will answer in french, if I answer in French, they switch to their dialect. Well I do understand that dialect., too

Some people are just not willing to talk to you because you are an alien ...but some don´t care about the language and are open to you.
I can tell you from own experience that speaking the local language is a plus, but you can communicate without speaking the local language in many ways.
I carry an offline translator on my phone, and I carry a booklet with pictures in it to show what I mean.
It brought me around the world without starving...
If you travel east, german is widely understood (i.e. Hungary and Romania (their ancestors came from Germany undreds of years ago and the german culture and language is an integral part of their lives), or Turkey, as many turkish people have worked in Germany.) Further east it´s russian, which will be understood by most of the people in Central Asia and the former USSR. English isn´t spoken by many.
And you might find fellow travellers who do speak the local langage...
Good luck and enjoy!