1. There are not categories for the user. You write in the plugin: "Input comma seperated list of internal categories (e.g. General, Movies, Family)." I tried some but no category appears...
This is only if you're using the Internal model. Otherwise you need to configure a Section further below and it'll use the categories of that content section. Please review the information directly next to the parameters.
2. What can I do to translate the frontend for the user-blogs to german?
Plugin is designed using complete language strings. You can translate by adding the translations to the cbteamsplugin file. If you need to retrieve the strings I recommend reviewing the AWK Script on the incubator which can be used to retrieve all the language strings.