On a completely utilitarian basis, if you already know English, I’d literarily recommend any other major European languauge than German.
Germany ranks very high in English proficiency
There are very few countries in Europe that speak English better and more widespread than Germans do. If you know English, you can make do with that in Germany and most texts you might be interested in German are often available in English too. If you aren’t planning to move there to work you’ll hardly need it.
French is the better option, although the French generally do have passable knowledge of English - they just aren’t keen on using it. Italian and Spanish are even better choices, because for some reason their knowledge of English is lacking, if we’re polite. Portugese is a surprisingly good option too, because Brazil is popolous - although I’d recommend Spanish well before Portugese.
In the east, Polish is an obvious option, but so is Russian. East of Germany, you will often be able to make do with Russian with people who don’t know English. This is fading though, especially after the 2022 invasion of Ukraine Russian will likely be on the way out, so it’s low on my list of recommended languagues (but still above Czech or Slovak or Slovene for their limited utility). Nonetheless, for use in Europe, Russian may well have more utility than German, in no small part because Slavic languagues are surprisingly close to one another and knowing one gives you a limited ability to communicate all across the east. This also works with Latin languagues (Italian, Spanish, French, Portugese) in the west.
If you’re moving to Germany (or Austria, or Switzerland), you should study German langague, there is no question about that. However if you just want a European languague for purpose of travel or tourism or just are looking for a useful languague as a part of general education, opt for something else instead.
