You have to use ISO language codes for external subtitles (.en, .fr, etc)
See Local Media Assets - TV Shows
TV Shows/ShowName/Season xx/ShowName - SxxEyy.[Language_Code].ext
Where [Language_Code] is defined by the ISO-639-1 (2-letter) or ISO-639-2/B (3-letter) standard.
Re-scan the show after correcting the subtitle file names. If they still do not appear, you may have to Plex Dance the episode, including subtitle files (or the entire season or show if that is easier).
