The way it works is that you can have many different language packs installed, and users can individually choose which one to display, so you might have 3 different languages being used simultaneously. It's not a system-wide setting.

Is there some way to have hierarchical keys for the cache? It would be nice to be able to do things like "invalidate all keys where lang=fr" (when a new language pack is downloaded) or "invalidate all keys where path=/math/algebra/" (when a new video is downloaded), without having to loop over all values of the other component of the key.