Add subtitle language code for external srt without them

[Rouzax]

Is your feature request related to a problem? Please describe.
There is an option to treat embedded subtitles as desired language when they are not marked. But it would be helpful if I could do the same thing for external srt files.

Example the following files the language code is not added during post process:

• The.Goldbergs.2013.S06E04.Hershey.Park.1080p.AMZN.WEB-DL.DDP5.1.H.264-NTb.mkv
• The.Goldbergs.2013.S06E04.Hershey.Park.1080p.AMZN.WEB-DL.DDP5.1.H.264-NTb.srt

Describe the solution you'd like
I want that srt file to be processed as English and adding the language code.