Add another line at the end of the .srt file with Subtitle Edit with as example a text of the language: ENGLISH (or anything else) worked for all my subtitles.
Or, when not using Subtitle Edit, open .srt file in Notepad and add another line at the bottom with a new timecode, for example 2 seconds later and duration also 2 seconds.
This becomes the last line then, and Plex does not show this line, but yes, Plex shows the last line of the spoken language.