Install the codestyling localization plugin.
Go to "Werkzeuge/Lokalisierung" and scroll down to Woocommerce subscriptions Plugin.
Click on "Neue Sprache hinzufügen" choose the german language and create it. Then click on "einlesen" on the right.
Now open with your FTP program the following folder on the server:
Upload my translated files to this, overwrite any existing
If your Wordpres is set to "German" de_DE, then the subscription should automaticall after a relóad be in German language.
Please reminnd, if the subscription plugin is updated via automatic update, the language files will be deleted. So just update and then re-upload the files again. If you make changes, please copy them to your hard disk before, so you have it on a save place to reupload :)
Hope this helps!