Any possibility to get hold of the data (name and log-in password, under which I contributed), as well as to get to the pages, where open projects (especially translations from articles from Wikipedia-Sites in other languages) are listed, since I am German, especially on the German Web-Site?
I do not really understand your question, and especiall not what "the pages where open projects are listed" refers to. Are you searching for a list of the Wikimedia projects, or something else?
If you have forgotten your username, the best way to find it is probably in the history in one of the pages you have edited. In case those are difficult to find, maybe you can tell the English title of some pages you translated or German translated titel, and we can help you find it.
If you have forgotten your password, you can get a new one - provided you put your e-mail address in your preferences, and authenticated it. If not, you will probably have to get a new user account - or edit non-logged in, as some people do. Interactions with others are easier if logged in, though.