If you want me to proof-read your English input, just ask. It's no problem. We have many non-native English speakers here. As long as your input is easily understood it doesn't matter if the grammar isn't perfect. I'll help any way I can, though.
As for the image, you need to upload the original German one again, but with the name Apper-Window/de. That then will be referenced from the German page.
Then upload your new one as Apper-Window. That name should be used on the English page.
When you are happy that all is in order, ask me and I'll remove the original apper-fenster image.
I hope that's clear :-) but if it isn't, ask again.
Thanks for your willingness to help me ;-)
I still have a question about naming the uploaded files: i can't name the image Apper-Window/de, right? should i call it Apper-Window_de.png (underscore) or should i use a hyphen? Is there an automatism that cross-references the original- and the translated picture-names ? => then i should be precise in name-giving
My mistake, sorry. You are quite right to call it Apper-Window_de.png. When it comes to this sort of question one way to always get reliable examples of good usage is to look at Danish or Ukranian pages, to see how they do it. Claus_chr (da) and Yurchor (uk) are prolific and very skilled. There's no automatic way of getting these image translation names. Following a mentor is your best bet, and these are your two most reliable sources, although there are several other prolific translators. You could also try leaving a message for Sordon - I assume you have already seen http://userbase.kde.org/Translation_Workflow/German :-)