One quietly motivating fact about German: it doesn't just give you German.
It sits in the middle of the Germanic family, so learning it hands you partial access to several neighbouring languages — especially in reading, where you have time to puzzle things out.
Dutch and Afrikaans: the closest cousins
Dutch sits right next to German, and German speakers can often follow a surprising amount of written Dutch — the vocabulary and structure rhyme. Afrikaans, which grew out of Dutch, is even more approachable on the page for many learners, with simpler grammar.
You won't be fluent, but a menu, a sign, or the gist of an article is often within reach.
Yiddish: closer than you'd expect
Yiddish has a strongly Germanic base and shares a great deal of everyday vocabulary with German. The main barrier is the script — traditionally written in Hebrew letters — but transliterated or spoken Yiddish is often startlingly understandable to a German speaker.
The Scandinavian languages: partial credit
Norwegian, Danish, and Swedish share Germanic roots and a lot of cognates with German. You won't read them fluently off the back of German alone, but you'll catch far more than someone starting cold — and if you ever study one, you'll have a running head start.
English, seen from underneath
You already speak English, but German shows you the older Germanic layer beneath it — the shared roots (water/Wasser, house/Haus, good/gut) that explain why so much English vocabulary suddenly looks familiar. Learning German quietly deepens your sense of your own language.
A key to a neighbourhood, not just one door
None of this makes you instantly multilingual — mutual intelligibility is partial, and mostly helps with reading and listening rather than speaking. But it does mean German is an unusually good investment: you're not learning one language in isolation, you're getting a key to a whole neighbourhood.