[chrissix666]

But I have alot of old .avi files on my computer that I wanted to run through media companion and import into kodi. 
Everything is working perfectly except that it obvisiosly can't detect the correct audio language as it isn't defined in the file (I have smart playlists that filter on language in kodi). 

This is normal behavior, also Kodi itself cannot detect and read all Media Tags of .avi files and other legacy formats.
Media Tags = Video Codec, Video aspect ratio, Audio Codec, Audio Channels, Audio Languages, included Subtitle Languages
It's a bit better working with .mp4, but most best working with .mkv. That's one of the reasons why there is almost only .mkv content you get from web.
 

I know I could either try and write a script that tags every file correctly

I really not recommend to do that! If you play once a scraped movie or episode in Kodi (only just for one second) all Media Tags will be updated and overwritten in your .nfo.
If you ever have the feeling there are scraped have wrong media tags or tags are missing, just push short the play button in Kodi and you will get the best Media Tags results you can get.

I also have pretty much old .avi stuff where i can't find new .mkv sources, regarding the Media Tags it's not nice i know, but you're going to have to deal with it.
As far as possible try go get always .mkv content from release groups you can trust that they doing there conversion/encoding/tagging stuff nice.