As far as I know there's no official way of adding culture specific properties to MediaData content.
For example: I would like to add a culture specific description and alt text to an image and I'd like to translate that.

Do you have any suggestions on how to do this, is support for this on the roadmap somewhere?

I know about these options but I was wondering if there's any better alternative (as I don't think media is indexed in different languages, for example): 
https://www.patrickvankleef.com/2014/06/16/use-culture-specific-mediadata-properties/ 
https://gregwiechec.com/2015/07/localizable-media-assets/ 
https://support.episerver.com/hc/en-us/articles/360000495192