Language translations are supposed to be automated using WordPress’ language system, unfortunately, after the last update in which I removed the local language files in favor of WordPress language system, it appears to be having issues.
The language packages get installed when the plugin is activated, you might be missing those language files. Please try de-activating and re-activating the plugin to see if it installs the language files.
If that does not work, please provide the exact German language you selected from the language options.