I’m planing on backpacking across Europe next year and I hate being the dipshit who only speaks one langue. I already have a small amount of german and spanish so those are the only two I’m interested in. I get the impression that outside of Spain spanish isn’t spoken much but I do plan on traveling south america at some point too so spanish is going to be needed eventually.
You’ll almost certainly be fine with only English because so many people speak at least a little bit of it. You may have trouble in places that don’t get many tourists but translate apps are pretty good these days and you can download language packs usually for offline translation. If you’re going to go to South America having a bit of spanish will be way more useful because at least in my experience english proficiency is a lot lower there than in Europe. So if you want to learn a language I’d say go with Spanish instead of German. You’ll get more practice by sticking to it longer, it’s probably more familiar to you if you’re an American, and it’ll overall be more useful for the kinds of things you want to do.