Well you quoted a post from a completely different thread and also mentioned BM here, so, without knowing the context, that confused me a tiny little bit xD
I do have a proofreader, like everyone writing a story should have. A single person wont ever find every typo, wrong wording, error, no matter how hard you try and how often you check the script. You'd be surprised how often I find tiny little errors in some of the oldest parts of my game, even though that has been checked and seen by thousands of people by now, and some of them are very eager to mention every little error they notice^^
What do you mean by german slangs? There are slangs in english as well, so if you write in english you use english slangs. If you mean idioms, that's a different story, but english and german have a surprising amount of similar idioms, google is a good place to find help with that if necessary.
I might be german, but who knows for sure