I am not sure if this is common to know, but I just found this out by accident. To use external subtitle files, you name your .srt file exactly the same name as your video file. However, you may notice under subtitle tab, the language is read 'unknown.'
I found out a way to have your language file(s) properly name. This works great if you have several different languages under the same movie title. So instead of naming the exact name as your movie file, do this instead.
Big Hero 6.mkv
Big Hero 6 English.srt
You will see 'English' instead of 'unknown' now.