:) unless you are able to adapt the file product-feed.php (found in the plugin directory) to the German taxonomy needs, i would disable the plugin and use the one detailed on my site, yes. The one in my site already has german taxonomy in it. (it's not a plugin thou and requires minimum php knowledge!)
I will start working on a German version of the plugin (with the right taxonomy for the German google merchant) tonight.
Sorry about this, all my customers for the implementation were US/UK/AUS so the plugin was aimed at helping them (also to show gratitude for their support) ;
You are the first person from DE that manifests interest into this :)
I'll start coding now...
Zum Wohl