Hi,
Create a main product category "german" and a main category "english". Then, create sub categories under each language category.
Add your products under these categories, either under only the english categories tree, or only the german categories tree, or both.
Then, change your products / categories listing menu items, you use these categories, and restrict the menu items per language:
docs.joomla.org/Help5.x:Menu_Item:_New_Item#Associations
That way, you'll be able to have different products / categories for different languages on your website.