Hi @lluisribes,
I am not sure I understand.. if the goal is to display closed captions [subtitles] in multiple languages then there is no need to use the custom metadata APIs at all.
What you need for that is the captions API. You can see how to work with that here:
https://developer.kaltura.com/recipes/captions#/start
If you wish to add captions using KMC, rather than using the API, you can add captions by going to the entry's drill down screen [click on the entry name on the "content" tab and go to to "Captions" on the left pane.
Regardless, if you're interested in learning how to work with the metadata APIs look here:
https://developer.kaltura.com/recipes/metadata#
But I won't recommend using that to store captions, since the caption APIs are specifically meant to support this and of course are used by the player as well.