there is currently no other way to show the language of all audio & subtitle streams of a video file than adding the language(s) to the filename and display images with substring conditions. Since XBMC already extracts all those infos (we just don't have the necessary info labels), I decided to make a small script which sets properties for the languages in the video info dialog.
You have to add this to DialogVideoInfo.xml
<onload condition="System.HasAddon(script.videolanguage) + [Container.Content(movies) | Container.Content(episodes)]">RunScript(script.videolanguage,movieid=$INFO[ListItem.DBID])</onload>
Then you can access the following properties in the video info dialog
I did some quick testing and it seems to work fine but you never know. Suggestions are welcome.