Have you read https://docs.joomla.org/J3.x:Setup_a_Multilingual_Site
Presently we are working on a major jD update that supports Multilingual Associations.
jDownloads has Categories and Downloads. This is just like Joomla! Categories and Articles. In each case you need a Category specific to each language That is if I set up a category called 'general' with say Spanish as its language and I create another category called 'general' but with German as its languge then these are two different categories. Of course one would in practice also translate the title.
If you have not already done so I would suggest you experiment with Joomla! articles.
In jDownloads Backend if you select an existing Category then you can use the Batch operation to copy a category and at the same time change its language. Would Suggest that you create the Spanish and German top level categories first. You need to go to each category to translate its title.
Sometimes the file that is to be downloaded is the same in both languages. One still needs to create separate Downloads, each with their language assignment but one of the Downloads can have the actual file and the other can use the 'Select from Other Download' feature.