If a plugin is not translated correctly or only partially, it is not always an error behind it. The prerequisite is that this plugin has been translated at all. In this case there is a subfolder „language“ in the corresponding plugin folder. The corresponding German translation file must then also be in the correct „language“ directory (/wp-content/language/) or copied there manually.
Special feature: In the German language WordPress can be set not only in German, but also in the German you form. This has the advantage if you do not want to address users with Du in standard output. The desired plugin may not have this Sie form and must then be adapted manually, but so that in this case the translation works at all, the file name must be extended: PLUGINNAME-en_DE_formal.FILEENDING