How to change the text of automatic E-Mails e.g. after changing your password? (German localization)

I found a way to manually edit the language files using Poedit. You simply download the .po-file, open it with the editor and you can see and edit all the messages created by WordPress, be it in an Email, a popup window…
The editor can then create a new mo.-files. The .po- and .mo-file then get uploaded (overwriting the existing files) and everything should be fine.

I guess you’ll have to the same process again after you intall a WP upgrade and I don’t know if you can simply use the already modified file for the new WP version. But for now this works fine.