[_J@n_]

Excellent idea that put me on the right track. I found a somewhat different solution that works for me.

Almost all my videos have English language as default and the subs (internal or external) are all in English as well. So I modified below settings:

Player -> Language -> Preferred audio language: English
Player -> Language -> Preferred subtitle language: English

Now KODI displays the external subs as default and when they are missing falls back to the internal subs.

Note that I didn't have to change the tracks inside the MKV to achieve this.

So happy camper!

Thanks for your support!

Greetz,

_J@n_