I think expecting users to tag things reliably or at all is tough, especially on mobile.
I would like to see auto/default tag settings on a per category basis as discussed at Topic auto tagging
Could be used to group the branches and/or languages by region for each category. I have a similar org structure but want tags for subcategories with events and galleries so the topics can all roll up across the org.
Using private groups really locks things down and could limit a lot of good discussions/searching.