Linguistic diversity is an amazing beautiful incredible thing. There are things you can say in English that you can't say in German, and vice-versa, or Portuguese, or Swahili, or Mandarin, or Lojban.
You'll forgive me for speculating that you don't speak more than one language, or you would appreciate that.
But. I do not complain about being there not being enough demand for any of the languages I speak. Demand is demand, if there isn't great demand for German-language novels, it is not the end of the world.
The answer to the question "I want make a living writing novels, but I am writing them in German and there isn't enough demand for them to really sustain myself - what should I do?" is not "force Germans to start reading more books in German instead of English". It's "learn to write books in English".
Do you really find whining about the fact that native Germans prefer to read books in English over his/her mother tongue constructive?
100 Years of Solitude is an absolute masterpiece. It's very difficult to put a finger on exactly why, but all agree there is a tone to it and to the way he tells the story, which usually people call "magical realism". The way it's so inexorably connected to a South American place and time, it's gonna be a different thing if you read it in English (credit though to the translators who do this incredibly difficult job in a stunning manner). The musicality and mood of the speech, for lack of a better description, cannot be accurately rendered in English.
In short, it's more nuanced than "what's a sentence I can say in English that I can't say in German".
Likewise from German there's not a direct translation for Backpfeifengesicht, a 'face that is asking to be slapped', more literally 'face slap face'. Is there a more Englishy way to say Zeitgeist that captures the same meaning? You could replace it with "spirit of the time", but it feels a bit too general and clumsy. Is there a succinct English equivalent to Schadenfreude that isn't too long or clumsy?
I don't know about Backpfeifengesicht, but both zeitgeist and schadenfreude are English words now.
Conversely in English stain has to be distinguished in cotext from the words for shirt stains, wood stains (Holzbeize) and stained glass (Buntglas).
Perhaps a better word to demonstrate it is Gemütlichkeit. [1]
Zeitgeist and Schadenfreude are exactly words imported into English because no words in English came close to capturing these words' meanings. You'd have to say "pleasure derived from another's misfortune" every time.
[0] https://www.dict.cc/deutsch-englisch/Fleck.html
[1] https://en.wikipedia.org/wiki/Gem%C3%BCtlichkeit#Similar_wor...
Many social constructs don't translate well. The word "freunde" is much, much stronger than "friends", for example. "We're all friends here" doesn't really work in German, at least not in the same situations. Similarly, English doesn't really have a term for "Wohnunggemeinschaft."
Puns often don't translate, which breaks translating many of the jokes in Shakespeare, for example. Going from German to English, the Rammstein song "Du has(s)t much" doesn't translate due to the puns, as does much of the writing by Yoko Tawada.
Gender ambiguous things don't always translate either. The gender neutral pronoun "they" translates to "sie" in German, which is the same as the pronoun "she" but with different verb conjugations. "Trans man" or "transwoman" are usually translated in German the same as "man" or "woman". Nuance on racial issues also doesn't always translate smoothly between the two languages.
I am sure there are many, many more.