Oh, I wasn’t suggesting you take up the writing gig. Just explaining how some common English colloquialisms wouldn’t make much sense at all if directly translated into German, and vice-versa. Just Googling a list of German colloquialisms, I got a bunch of stuff that would sound absurd to an American – things about tying a bear on someone, or splitting a bear pelt before the animal is skinned (apparently a version of “don’t count your chickens before they hatch”).
Especially if the story is about teenagers, you’re going to need someone well-versed in slang in both languages, if you’re insisting on having English-speaking people to comment on your ultimately German story. Teenagers typically have a highly vernacular way of speech, so writing in a formal, easily translatable way isn’t really going to have the right feel.