If you literally mean she has physical skeletons in her basement, your way is best. The phrase "to have skeletons in your closet" means someones has secrets and is extremely common here. I assumed you wanted the phrase and changed it for you. But change it back if you are referring to actual skeletons. Then wordplay doesn't apply.
I meant "to have skeletons in your closet" as "to have secrets"
but in german this phrase could also end with basement instead of closet, so it is a perfect match
in the online translation tool I have used, there I also found an english version featuring basement, so I thought I could use that instead
too bad it does not work

There was a sentence on the poster next to the apartments, "It's not allowed to placard here!" This wording won't really convey what that means very well. I know you mean something along the lines of "Don't advertise here." but I would suggest a rewrite of that poster for us Americans. 
actually there are 2 posters. the left one should say something like "Do not put posters on this wall".
the joke is, that someone put a poster right beside it that says "Okay!"
so there are 2 posters on a wall where no posters are allowed

The address on the building is 4a. Is that how addresses are in Europe? With numbers and letters? We don't really do that on the outsides of our buildings. Our apartments inside the building are commonly done that way, but not addresses.
this background graphic is from an other episode done by Mister L and so it became part of Ronville
I just used it but did not change anything on it

but I think I will keep it the wrong way, since there are a bunch of episodes featuring this background with 4a
so at least we will all have it wrong

I've never seen anyone sign a letter with the closing "sgd:" before. I assume it means "signed." Perhaps that's more common in Germany? You could American-ize it to a simple "Sincerely" or "Regards". I didn't change it. Your way probably works fine. Just wasn't sure.
yes, this means "signed" (got it from the online translation tool)
actually that isn't common in Germany either

but I have seen that a couple of times on TV when someone reads a letter aloud, so I got used to it

There's a line "I'm not a standard bearer" at some point in the game. I couldn't figure out what you were intending with this. Perhaps a rewording would help. I can help if you'd like with a little explanation.
that is the response when you try to pick up the banner at the police picnic
once again, I got it from the online translation tool
it says that a "standard bearer" is someone who carries a banner
you know it from knight-movies. there is always one guy carrying a banner while all the others are fighting

I should have reworded the line "Where's that ground coming from?" but with only the text in front of me, I couldn't remember the context. That's one I'd like to see again during gameplay to see how it works.
at two points in the game, you suddenly can interact with the floor you are standing on (you can put a bucket on it or dig a hole in it)
but there was no hotspot before, so Klaus is wondering where this hotspot is coming from

this is the reaction when Klaus looks at the ground
Ok, well that's the fun in translation! Those were all my notes for about 50 pages of story. Pretty great considering, I think, on your part. Translating isn't easy. I hope I'm being of some help to the overall game. I'll finish up the rest by tomorrow.
you are a great help!
at least I did not get the urgent need to bang my head on the wall, when I played the first 5 minutes using the proofread translation

my original translation did that
