Enable external subtitles for extra local content

It would be really useful to be able to enable external subtitles for extra local content. At the moment this function is not available, but it is very important, especially for all those who do not understand English.

Thank you.

@OttoKerner said that this functionality is not possible in here Local subtitles for extra movie files (interviews, deleted scenes, etc). I REALLY would like to understand why. Seems arbitrary to me. Is very convenient, and spare time for the users to add this support. I know that using a mix tool like MKVToolNix is a way to get arround this limitation, but honestly why this is not implemented, or even “not possible” idk.