[mYnDstrEAm]

It would be nice if one could easily filter displayed movies/series if they're available in a specific language / the configured preferred audio language in the settings.

For example, one may have lots of German-language movies but sometimes would like to have Kodi only display the fraction with English audio tracks...one could go to the settings and configure English as preferred audio language, then there could be a toggle option in the left sidebar for "Show only if available in preferred audio language".

Alternatively or additionally, there could be a small colorful icon in a corner of the cover that shows if the media is available with the configured preferred audio track language.

Somewhat related: it would be best if at some point one could conveniently quickly switch between a few preferred languages in the options, a kind of quick-access option where one can toggle between the preferred languages (like you can already do for individual movies when you press the gear icon in the bottom right), but that's probably something for later.

I don't know if this is already possible somehow but I couldn't find a filter for languages under Filters in the left sidebar of the Movies/Series page nor could I find info in the wiki here or here.

[Karellen]

one may have lots of German-language movies but sometimes would like to have Kodi only display the fraction with English audio tracks

Use a Smart Playlist... https://kodi.wiki/view/Smart_playlists

[mYnDstrEAm]

Use a Smart Playlist... https://kodi.wiki/view/Smart_playlists

Thanks, however besides that it's not as convenient as a filter option in the left panel and probably not easily found / known by most:

that doesn't work with series. 
You can configure a filter for Movies, setting audio track languages to is "eng", but there's no option like this for this for series. You can only configure that for episodes. That displays a page full of tiles of episode from all your series instead of a page with covers for series which have any episodes with English audio tracks.

[DiMag]

If this gentleman is right (and it seems he is) this is a serious omission and it must be rectified. Of course TV series have a property "spoken language"! --- who has heard of a Babel-esque series with spoken language differing on each episode?

As an aside there is a quick & dirty solution until this part of the library is fixed: Tag your serie either with <genre>TV DE</genre> or with <tag>TV DE</genre> (or with both, why not?) and then filter it in a playlist. I do it all the time with success, though my aim is to distinguish British-made and French-made series from the US lot. My aim is to filter not per language but on style and production quality; though of course it filters language as well.