Taxanomy & new navigation

Hi,

I hope you can help me. My questions is about creating the right navigation on my educational site.

On my site I have created different categories like:
– Math
– History
– Biology
– Ect.

For each category I have added sub categories. For “math” I have added:
– Algebra
– Calculus
– Geometry
– Ect.

Because the site will contain lots of content I would like to “tag” each post with two new taxanomies called “Age” (because some of the content is targeting primary school children and other content is targeting high school) and “Language” (Because some post will contain videos with english and other will be recorded in german, spanish ect)

I have tried to simple add tags but this is not working because it will find all let’s say sanish lessons in all categories and not only the one category first picked – for example Algebra.

I have all ready installed a plugin which make it easy to add new taxanomies but this just add a new kind of tag or category.

What I need is a way to sort/filter the categories in “Age” and “Language” after the user has picked a category.