information architecture – How can I simplify this highly nested data structure?

Trying to layer multiple levels of tab controls can get messy, fast.

I would suggest you stick with something more simple such as a navigation menu. You could also make it an accordion menu if you don’t want all sub-categories to be visible at the same time.

Here is an example of a simple menu:

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It isn’t clear what “medium” and “language” are in your question. So this idea works on the basis that they are field sets and fields. Therefore, you have the navigation menu for categories, and then a single (scrollable) view that will hold all the data for the selected sub-category.