Smart Libraries (or at least a filter for audio language)

I got my parents a Roku Stick and access to my Movie library, but most of my movies are in english. It would be nice if I could create a library that only contains movies in german.

 

The most comfortable way would be if I could specify a language filter while creating the library. I guess that's a bigger change, so it would be sufficient if there would be a way to filter by audio language (in the regular movie library interface).

early 2021 clean-up: implemented (smart playlists, filter by audio language)