[henricos]

.... this has been fixed for the upcoming v15.1: https://github.com/xbmc/xbmc/pull/7597
This automatically adds languages that are installed in Kodi to the subtitle options.

You guys are the best!

Thank you very much @Ace , @Ned Scott and all Kodi developers who helped with this fix.

[Ned Scott]

no need to thank me, I didn't do anything ;)

Ace's solution is very clever and takes care the whole debate about which ones to add or not, since it just reads that off of what language files are installed.