I found it. Under the "Advanced" button there is an option "Multilingual content setup". When I tick "Make 'Products' translatable" I can indeed translate my products.
Does your shop really work? My products aren't distinguished between two languages. I see always both products in the store. And how you managed to translate editable labels like the Purchase-Button from Stripe-Add-on and the three titles of Objectives, Intended Audience and Prerequisites? And also the titles of pages under settings, there is only one name for each site...